Further enhance, remove the offset, and add in another new algorithm that'll always and almost instantly face the back of the humanoidrootpart and will always maintain it's lock unless u lock-on to another target or unlock.

```
--[[
    Enhanced Lock-On and ESP Script with GUI for Body Part Selection and Layered Tracking
    - Allows selection of specific humanoid body parts: Left Arm, Left Leg, Torso, Torso (Back), Head, Right Leg, Right Arm.
    - Implements a layered tracking method to prioritize and manage target selections based on Field of View (FOV) and distance.
    - Integrates a user-friendly and visually appealing GUI for real-time body part selection.
    - Maintains all previous functionalities including ShiftLock support, predictive algorithms, and hitbox detection.
    - **Enhancements:**
        - Advanced Torso (Back) Tracking
        - Robust Movement Prediction using Kalman Filter
--]]

-- Services
local Players = game:GetService("Players")
local RunService = game:GetService("RunService")
local UserInputService = game:GetService("UserInputService")
local GuiService = game:GetService("GuiService")
local TweenService = game:GetService("TweenService")
local Workspace = game:GetService("Workspace")
local CurrentCamera = Workspace.CurrentCamera

-- Configuration
local DEBUG = true  -- Toggle debug logs
local ROTATION_SPEED = 25        -- Smooth rotation speed multiplier (reduced for smoother effect)
local ESP_TEXT_PREFIX = "Targeted: " -- Prefix Text displayed in ESP
local ESP_COLOR = Color3.fromRGB(0, 255, 0)  -- Color of the ESP text
local PREDICTION_TIME_BASE = 0.2 -- Base time in seconds to predict target movement (reduced for responsiveness)
local PREDICTION_TIME_ADJUSTMENT_FACTOR = 0.3 -- Factor to adjust prediction time based on velocity and distance
local VELOCITY_ALPHA = 0.8 -- Alpha value for Exponential Moving Average (now replaced by Kalman Filter)
local LOCK_ON_KEY = Enum.KeyCode.E -- Key to toggle lock-on
local SHIFT_LOCK_TOGGLE_KEY = Enum.KeyCode.LeftShift -- Key to toggle ShiftLock mode
local BACK_OFFSET = 3 -- Studs behind the target's torso to aim (reduced for accuracy)
local MAX_FOV_ANGLE = 60 -- Maximum angle (in degrees) for FOV-based target selection
local GUI_CONFIG = {
    ScreenGuiName = "EnhancedLockOnESPGUI",
    FrameSize = UDim2.new(0, 220, 0, 350),
    FramePosition = UDim2.new(0, 10, 0.5, -175),
    ButtonSize = UDim2.new(1, -20, 0, 35),
    BodyParts = {
        "Left Arm",
        "Left Leg",
        "Torso",
        "Torso (Back)",
        "Head",
        "Right Leg",
        "Right Arm"
    },
    SelectedColor = Color3.fromRGB(70, 130, 180), -- Steel Blue
    UnselectedColor = Color3.fromRGB(50, 50, 50),
    HoverColor = Color3.fromRGB(100, 100, 100),
    TextColor = Color3.fromRGB(255, 255, 255),
    BackgroundColor = Color3.fromRGB(30, 30, 30),
    StrokeColor = Color3.fromRGB(255, 255, 255)
}

-- Debug Function
local function debugLog(message)
    if DEBUG then
        print("[EnhancedLockOnScript]: " .. message)
    end
end

-- Variables
local player = Players.LocalPlayer
if not player then
    error("LocalPlayer not found!")
    return
end

local character = player.Character or player.CharacterAdded:Wait()
local humanoid = character:WaitForChild("Humanoid")
local humanoidRootPart = character:WaitForChild("HumanoidRootPart")
local trackedTarget = nil          -- Currently tracked target (another player)
local espIndicator = nil           -- BillboardGui for ESP
local shiftLockMode = false        -- ShiftLock mode toggle
local lockOnMode = false           -- Lock-On mode toggle
-- Remove velocityEMA as it's replaced by Kalman Filter
local selectedBodyPart = "Torso (Back)" -- Default selected body part

-- Determine Rig Type and set the torso part name
local isR15 = humanoid.RigType == Enum.HumanoidRigType.R15
local torsoPartName = isR15 and "UpperTorso" or "Torso"
debugLog("Rig type determined: " .. (isR15 and "R15" or "R6"))
debugLog("Torso part set to: " .. torsoPartName)

-- Mapping for R6 and R15 body parts
local bodyPartMappings = {
    ["Left Arm"] = isR15 and "LeftUpperArm" or "Left Arm",
    ["Left Leg"] = isR15 and "LeftUpperLeg" or "Left Leg",
    ["Torso"] = torsoPartName,
    ["Torso (Back)"] = torsoPartName, -- Special handling
    ["Head"] = "Head",
    ["Right Leg"] = isR15 and "RightUpperLeg" or "Right Leg",
    ["Right Arm"] = isR15 and "RightUpperArm" or "Right Arm"
}

-- Helper Function: Check if a model has a Humanoid
local function hasHumanoid(model)
    return model and model:FindFirstChildOfClass("Humanoid") ~= nil
end

-- Helper Function: Calculate angle between two vectors
local function calculateAngle(direction)
    local viewVector = CurrentCamera.CFrame.LookVector
    local directionUnit = direction.Unit
    local dotProduct = viewVector:Dot(directionUnit)
    local angle = math.acos(math.clamp(dotProduct, -1, 1)) * (180 / math.pi) -- Clamp to avoid NaNs
    return angle
end

-- Kalman Filter Implementation for Enhanced Prediction
local KalmanFilter = {}
KalmanFilter.__index = KalmanFilter

function KalmanFilter.new()
    local self = setmetatable({}, KalmanFilter)
    self.position = Vector3.new(0, 0, 0)
    self.velocity = Vector3.new(0, 0, 0)
    self.acceleration = Vector3.new(0, 0, 0)
    self.dt = 1 / 60 -- Default timestep, will be updated each frame
    return self
end

function KalmanFilter:update(measurement)
    -- Time update
    self.position = self.position + self.velocity * self.dt + 0.5 * self.acceleration * self.dt^2
    self.velocity = self.velocity + self.acceleration * self.dt

    -- Measurement update
    local error = measurement - self.position
    local adjustmentFactor = 0.5 -- Tunable parameter for responsiveness
    self.position = self.position + error * adjustmentFactor

    -- Estimate velocity based on the new position
    self.velocity = (self.position - (self.position - self.velocity * self.dt)) / self.dt

    -- Optionally, update acceleration if needed
end

-- Create a table to hold Kalman Filters for each target
local kalmanFilters = {}

-- Helper Function: Find or create a Kalman Filter for a target
local function getKalmanFilter(target)
    if not kalmanFilters[target] then
        kalmanFilters[target] = KalmanFilter.new()
    end
    return kalmanFilters[target]
end

-- Helper Function: Find the best target based on FOV and distance
local function findBestTarget()
    local bestTarget = nil
    local shortestDistance = math.huge
    local bestAngle = MAX_FOV_ANGLE

    for _, otherPlayer in ipairs(Players:GetPlayers()) do
        if otherPlayer ~= player and otherPlayer.Character and hasHumanoid(otherPlayer.Character) then
            local targetPartName = bodyPartMappings[selectedBodyPart]
            local otherTorso = otherPlayer.Character:FindFirstChild(targetPartName)
            if otherTorso and otherTorso:IsA("BasePart") then
                local direction = otherTorso.Position - humanoidRootPart.Position
                local distance = direction.Magnitude
                local angle = calculateAngle(direction)
                
                if angle <= MAX_FOV_ANGLE and distance < shortestDistance then
                    shortestDistance = distance
                    bestTarget = otherPlayer
                    bestAngle = angle
                end
            end
        end
    end

    if bestTarget then
        debugLog(string.format("Best target found: %s | Distance: %.2f | Angle: %.2f", bestTarget.Name, shortestDistance, bestAngle))
    else
        debugLog("No suitable target found within FOV.")
    end

    return bestTarget
end

-- Function: Toggle Lock-On to the Best Target based on layered selection
local function toggleLockOn()
    if lockOnMode then
        -- Deselect the current target
        debugLog("Deselecting current target: " .. (trackedTarget and trackedTarget.Name or "None"))
        trackedTarget = nil
        lockOnMode = false
        if espIndicator then
            espIndicator:Destroy()
            espIndicator = nil
            debugLog("ESP indicator destroyed.")
        end
    else
        -- Find and select the best target
        local bestPlayer = findBestTarget()
        if bestPlayer and bestPlayer.Character then
            trackedTarget = bestPlayer.Character
            lockOnMode = true
            debugLog("Tracking new target: " .. bestPlayer.Name)
            -- Create ESP indicator
            local targetPartName = bodyPartMappings[selectedBodyPart]
            local targetPart = trackedTarget:FindFirstChild(targetPartName)
            if targetPart then
                -- Ensure previous ESP is removed
                if espIndicator then
                    espIndicator:Destroy()
                    espIndicator = nil
                    debugLog("Previous ESP indicator destroyed.")
                end

                -- Create BillboardGui for ESP
                espIndicator = Instance.new("BillboardGui")
                espIndicator.Adornee = targetPart
                espIndicator.Size = UDim2.new(0, 150, 0, 50)
                espIndicator.StudsOffset = Vector3.new(0, 2, 0)
                espIndicator.AlwaysOnTop = true
                espIndicator.Name = "TargetESP"

                -- Create TextLabel for ESP
                local textLabel = Instance.new("TextLabel")
                textLabel.Parent = espIndicator
                textLabel.Size = UDim2.new(1, 0, 1, 0)
                textLabel.BackgroundTransparency = 1
                textLabel.Text = ESP_TEXT_PREFIX .. trackedTarget.Name .. " (" .. selectedBodyPart .. ")"
                textLabel.TextColor3 = ESP_COLOR
                textLabel.Font = Enum.Font.SourceSansBold
                textLabel.TextScaled = true
                textLabel.TextStrokeTransparency = 0.8

                -- Parent the ESP to the player's PlayerGui
                local playerGui = player:WaitForChild("PlayerGui")
                espIndicator.Parent = playerGui
                debugLog("ESP indicator created for " .. bestPlayer.Name .. " targeting " .. selectedBodyPart)
            else
                debugLog("Selected body part '" .. targetPartName .. "' not found on " .. bestPlayer.Name)
                trackedTarget = nil
            end
        else
            debugLog("No valid target to track.")
        end
    end
end

-- Function: Predict Future Position Based on Target's Velocity using Kalman Filter
local function getPredictedPosition(targetCharacter)
    local targetPartName = bodyPartMappings[selectedBodyPart]
    local targetPart = targetCharacter:FindFirstChild(targetPartName)
    if not targetPart then
        debugLog("Target does not have the part: " .. targetPartName)
        return nil
    end

    -- Get current position
    local currentPosition = targetPart.Position

    -- Update Kalman Filter with the current position
    local kalFilter = getKalmanFilter(targetCharacter)
    kalFilter:update(currentPosition)

    -- Get predicted position
    local predictedPosition = kalFilter.position

    -- Enhanced Handling for "Torso (Back)"
    if selectedBodyPart == "Torso (Back)" then
        local targetHumanoid = targetCharacter:FindFirstChildOfClass("Humanoid")
        if targetHumanoid then
            local rootPart = targetHumanoid.RootPart or targetCharacter:FindFirstChild("HumanoidRootPart") or targetCharacter:FindFirstChild("UpperTorso")
            if rootPart then
                -- Calculate dynamic back offset based on target's velocity and orientation
                local dynamicBackOffset = BACK_OFFSET + math.clamp(kalFilter.velocity.Magnitude * 0.1, 0, 5)
                predictedPosition = predictedPosition - (rootPart.CFrame.LookVector * dynamicBackOffset)
                debugLog("Calculated back position using dynamic LookVector offset.")
            else
                debugLog("Root part not found for back position calculation.")
            end
        end
    end

    debugLog(string.format("Final target position: (%.2f, %.2f, %.2f)", predictedPosition.X, predictedPosition.Y, predictedPosition.Z))
    return predictedPosition
end

-- Function: Instantly rotate to face the predicted target position
local function instantRotateToPosition(targetPosition)
    if not targetPosition then return end

    local desiredCFrame = CFrame.new(humanoidRootPart.Position, targetPosition)
    humanoidRootPart.CFrame = desiredCFrame
    debugLog("Instantly rotated to face target position.")
end

-- Function: Smoothly rotate to face the predicted target position
local function smoothRotateToPosition(targetPosition, deltaTime)
    if not targetPosition then return end

    local direction = (Vector3.new(targetPosition.X, humanoidRootPart.Position.Y, targetPosition.Z) - humanoidRootPart.Position).Unit
    local desiredCFrame = CFrame.new(humanoidRootPart.Position, humanoidRootPart.Position + direction)

    -- Smoothly interpolate the CFrame
    humanoidRootPart.CFrame = humanoidRootPart.CFrame:Lerp(desiredCFrame, ROTATION_SPEED * deltaTime)
    debugLog("Smoothly rotated to face target position.")
end

-- Function: Rotate to Target (Instant or Smooth based on toggle)
local function rotateToTarget(deltaTime)
    if not trackedTarget then return end

    local targetPartName = bodyPartMappings[selectedBodyPart]
    local targetPart = trackedTarget:FindFirstChild(targetPartName)
    if targetPart and targetPart:IsA("BasePart") then
        local predictedPosition = getPredictedPosition(trackedTarget)
        if predictedPosition then
            if shiftLockMode then
                instantRotateToPosition(predictedPosition)
            else
                smoothRotateToPosition(predictedPosition, deltaTime)
            end

            -- Hitbox Detection: Ensure predicted position is within part bounds
            local partSize = targetPart.Size
            local partPosition = targetPart.Position
            local deltaPosition = predictedPosition - partPosition

            local withinX = math.abs(deltaPosition.X) <= (partSize.X / 2 + (selectedBodyPart == "Torso (Back)" and BACK_OFFSET or 0))
            local withinY = math.abs(deltaPosition.Y) <= (partSize.Y / 2)
            local withinZ = math.abs(deltaPosition.Z) <= (partSize.Z / 2 + (selectedBodyPart == "Torso (Back)" and BACK_OFFSET or 0))

            if withinX and withinY and withinZ then
                debugLog("Predicted position is within the part's hitbox.")
            else
                debugLog("Predicted position is outside the part's hitbox.")
            end
        else
            debugLog("Failed to get predicted target position.")
        end
    else
        debugLog("Target part '" .. targetPartName .. "' is invalid or missing.")
    end
end

-- Function: Handle ShiftLock Toggle
local function toggleShiftLock()
    if shiftLockMode then
        -- Disable ShiftLock Mode
        shiftLockMode = false
        CurrentCamera.CameraType = Enum.CameraType.Custom
        debugLog("ShiftLock mode disabled.")
    else
        if lockOnMode then
            -- Disable Lock-On before enabling ShiftLock to prevent conflicts
            toggleLockOn()
        end
        -- Enable ShiftLock Mode
        shiftLockMode = true
        CurrentCamera.CameraType = Enum.CameraType.Scriptable
        -- Lock the camera to look at the target
        if trackedTarget then
            local targetPartName = bodyPartMappings[selectedBodyPart]
            local targetPart = trackedTarget:FindFirstChild(targetPartName)
            if targetPart then
                local predictedPosition = getPredictedPosition(trackedTarget)
                CurrentCamera.CFrame = CFrame.new(humanoidRootPart.Position, predictedPosition)
                debugLog("ShiftLock mode enabled and camera locked onto target.")
            end
        end
    end
end

-- Function: Clean up ESP when the tracked target dies or leaves
local function onTrackedTargetRemoved()
    if not trackedTarget then return end
    debugLog("Tracked target removed or died: " .. (trackedTarget.Name or "Unknown"))
    trackedTarget = nil
    lockOnMode = false
    if espIndicator then
        espIndicator:Destroy()
        espIndicator = nil
        debugLog("ESP indicator destroyed due to target removal.")
    end
    -- Reset Kalman Filter for the target
    kalmanFilters[trackedTarget] = nil
    -- Reset camera if ShiftLock was active
    if shiftLockMode then
        toggleShiftLock()
    end
end

-- Function: Monitor Tracked Target for Removal or Death
local function monitorTrackedTarget(targetCharacter)
    if not targetCharacter then return end

    local humanoid = targetCharacter:FindFirstChildOfClass("Humanoid")
    if humanoid then
        humanoid.Died:Connect(function()
            debugLog("Tracked target " .. (targetCharacter.Name or "Unknown") .. " has died.")
            onTrackedTargetRemoved()
        end)
    end

    targetCharacter.AncestryChanged:Connect(function()
        if not targetCharacter:IsDescendantOf(game) then
            debugLog("Tracked target " .. (targetCharacter.Name or "Unknown") .. " has left the game.")
            onTrackedTargetRemoved()
        end
    end)
end

-- Modify toggleLockOn to monitor the tracked target
local originalToggleLockOn = toggleLockOn
toggleLockOn = function()
    originalToggleLockOn()
    if trackedTarget then
        monitorTrackedTarget(trackedTarget)
    end
end

-- Function: Handle Character Respawn
player.CharacterAdded:Connect(function(newCharacter)
    debugLog("Character respawned.")
    character = newCharacter
    humanoid = character:WaitForChild("Humanoid")
    humanoidRootPart = character:WaitForChild("HumanoidRootPart")
    
    -- Determine rig type using RigType property
    isR15 = humanoid.RigType == Enum.HumanoidRigType.R15
    torsoPartName = isR15 and "UpperTorso" or "Torso"
    debugLog("New rig type determined: " .. (isR15 and "R15" or "R6"))
    
    -- Reset tracking and ESP on respawn
    onTrackedTargetRemoved()
end)

-- Function: Update Camera during ShiftLock Mode
local function updateCamera(deltaTime)
    if shiftLockMode and trackedTarget then
        local targetPartName = bodyPartMappings[selectedBodyPart]
        local targetPart = trackedTarget:FindFirstChild(targetPartName)
        if targetPart then
            -- Smoothly move the camera to follow the target
            local predictedPosition = getPredictedPosition(trackedTarget)
            if predictedPosition then
                local desiredCFrame = CFrame.new(humanoidRootPart.Position, predictedPosition)
                CurrentCamera.CFrame = CurrentCamera.CFrame:lerp(desiredCFrame, ROTATION_SPEED * deltaTime)
                debugLog("Camera updated to follow target in ShiftLock mode.")
            end
        end
    end
end

-- Function: Create and Setup the Body Part Selection GUI with Enhanced Aesthetics
local function createBodyPartSelectionGUI()
    -- Check if the GUI already exists to prevent duplication
    if player.PlayerGui:FindFirstChild(GUI_CONFIG.ScreenGuiName) then
        debugLog("GUI already exists. Skipping creation.")
        return
    end

    -- Create ScreenGui
    local screenGui = Instance.new("ScreenGui")
    screenGui.Name = GUI_CONFIG.ScreenGuiName
    screenGui.Parent = player.PlayerGui
    screenGui.ResetOnSpawn = false

    -- Create Background Frame
    local frame = Instance.new("Frame")
    frame.Size = GUI_CONFIG.FrameSize
    frame.Position = GUI_CONFIG.FramePosition
    frame.BackgroundColor3 = GUI_CONFIG.BackgroundColor
    frame.BackgroundTransparency = 0.8
    frame.BorderSizePixel = 0
    frame.Parent = screenGui

    -- Add UIStroke for Frame
    local frameStroke = Instance.new("UIStroke")
    frameStroke.Color = GUI_CONFIG.StrokeColor
    frameStroke.Thickness = 2
    frameStroke.Parent = frame

    -- Create Title
    local title = Instance.new("TextLabel")
    title.Size = UDim2.new(1, 0, 0, 40)
    title.Position = UDim2.new(0, 0, 0, 0)
    title.BackgroundTransparency = 1
    title.Text = "Lock-On ESP"
    title.Font = Enum.Font.GothamBold
    title.TextSize = 24
    title.TextColor3 = GUI_CONFIG.TextColor
    title.TextStrokeTransparency = 0.5
    title.TextXAlignment = Enum.TextXAlignment.Center
    title.Parent = frame

    -- Create UIListLayout for organized buttons
    local uiListLayout = Instance.new("UIListLayout")
    uiListLayout.Parent = frame
    uiListLayout.FillDirection = Enum.FillDirection.Vertical
    uiListLayout.SortOrder = Enum.SortOrder.LayoutOrder
    uiListLayout.Padding = UDim.new(0, 10)
    uiListLayout.VerticalAlignment = Enum.VerticalAlignment.Top

    -- Create UIListLayoutPadding to position buttons below the title
    local layoutPadding = Instance.new("UIPadding")
    layoutPadding.PaddingTop = UDim.new(0, 50)
    layoutPadding.Parent = frame

    -- Function to handle body part selection
    local function selectBodyPart(bodyPart, button)
        selectedBodyPart = bodyPart
        debugLog("Selected body part: " .. bodyPart)
        toggleLockOn() -- Deselect current target

        -- Update ESP text if a target is already locked on
        if lockOnMode and espIndicator then
            espIndicator:Destroy()
            espIndicator = nil
            debugLog("ESP indicator destroyed due to body part change.")
            toggleLockOn() -- Re-select target with new body part
        end

        -- Update button colors to indicate selection
        for _, btn in ipairs(frame:GetChildren()) do
            if btn:IsA("TextButton") then
                btn.BackgroundColor3 = GUI_CONFIG.UnselectedColor
            end
        end
        button.BackgroundColor3 = GUI_CONFIG.SelectedColor
    end

    -- Create Buttons for each Body Part with Enhanced Aesthetics
    for _, bodyPart in ipairs(GUI_CONFIG.BodyParts) do
        local button = Instance.new("TextButton")
        button.Size = GUI_CONFIG.ButtonSize
        button.Text = bodyPart
        button.Font = Enum.Font.Gotham
        button.TextSize = 18
        button.TextColor3 = GUI_CONFIG.TextColor
        button.BackgroundColor3 = (bodyPart == selectedBodyPart) and GUI_CONFIG.SelectedColor or GUI_CONFIG.UnselectedColor
        button.AutoButtonColor = false
        button.BorderSizePixel = 0
        button.Parent = frame

        -- Add UIStroke for Buttons
        local buttonStroke = Instance.new("UIStroke")
        buttonStroke.Color = GUI_CONFIG.StrokeColor
        buttonStroke.Thickness = 1
        buttonStroke.Parent = button

        -- Hover Effects
        button.MouseEnter:Connect(function()
            if bodyPart ~= selectedBodyPart then
                button.BackgroundColor3 = GUI_CONFIG.HoverColor
            end
        end)

        button.MouseLeave:Connect(function()
            if bodyPart ~= selectedBodyPart then
                button.BackgroundColor3 = GUI_CONFIG.UnselectedColor
            end
        end)

        button.MouseButton1Click:Connect(function()
            selectBodyPart(bodyPart, button)
        end)
    end

    debugLog("Enhanced Body Part Selection GUI created.")
end

-- Call the function to create the GUI
createBodyPartSelectionGUI()

-- Function: Setup ShiftLock and Lock-On Toggles
local function setupShiftLockToggle()
    UserInputService.InputBegan:Connect(function(input, gameProcessed)
        if gameProcessed then return end

        if input.KeyCode == LOCK_ON_KEY then
            toggleLockOn()
        elseif input.KeyCode == SHIFT_LOCK_TOGGLE_KEY then
            toggleShiftLock()
        end
    end)
end

-- Event Listeners

-- Setup ShiftLock and Lock-On Toggles
setupShiftLockToggle()

-- Render Step: Continuously update rotation and camera if a target is tracked
RunService.RenderStepped:Connect(function(deltaTime)
    if lockOnMode and trackedTarget then
        pcall(function()
            rotateToTarget(deltaTime)
            updateCamera(deltaTime)
        end)
    end
end)