/-- An auxiliary lemma that combines two “exponential of a power” estimates into a similar estimate
on the difference of the functions. -/
theorem isBigO_sub_exp_rpow {a : ℝ} {f g : ℂ → E} {l : Filter ℂ}
    (hBf : ∃ c < a, ∃ B, f =O[cobounded ℂ ⊓ l] fun z => expR (B * abs z ^ c))
    (hBg : ∃ c < a, ∃ B, g =O[cobounded ℂ ⊓ l] fun z => expR (B * abs z ^ c)) :
    ∃ c < a, ∃ B, (f - g) =O[cobounded ℂ ⊓ l] fun z => expR (B * abs z ^ c) := by
  have : ∀ {c₁ c₂ B₁ B₂ : ℝ}, c₁ ≤ c₂ → 0 ≤ B₂ → B₁ ≤ B₂ →
      (fun z : ℂ => expR (B₁ * abs z ^ c₁)) =O[cobounded ℂ ⊓ l]
        fun z => expR (B₂ * abs z ^ c₂) := fun hc hB₀ hB ↦ .of_bound 1 <| by
    filter_upwards [(eventually_cobounded_le_norm 1).filter_mono inf_le_left] with z hz
    simp only [one_mul, Real.norm_eq_abs, Real.abs_exp]
    gcongr; assumption
  rcases hBf with ⟨cf, hcf, Bf, hOf⟩; rcases hBg with ⟨cg, hcg, Bg, hOg⟩
  refine' ⟨max cf cg, max_lt hcf hcg, max 0 (max Bf Bg), _⟩
  refine' (hOf.trans <| this _ _ _).sub (hOg.trans <| this _ _ _)
  

Proof state:
case intro.intro.intro.intro.intro.intro.refine'_1
E : Type u_1
inst✝ : NormedAddCommGroup E
a : ℝ
f g : ℂ → E
l : Filter ℂ
this :
  ∀ {c₁ c₂ B₁ B₂ : ℝ},
    c₁ ≤ c₂ →
      0 ≤ B₂ →
        B₁ ≤ B₂ → (fun z => expR (B₁ * Complex.abs z ^ c₁)) =O[cobounded ℂ ⊓ l] fun z => expR (B₂ * Complex.abs z ^ c₂)
cf : ℝ
hcf : cf < a
Bf : ℝ
hOf : f =O[cobounded ℂ ⊓ l] fun z => expR (Bf * Complex.abs z ^ cf)
cg : ℝ
hcg : cg < a
Bg : ℝ
hOg : g =O[cobounded ℂ ⊓ l] fun z => expR (Bg * Complex.abs z ^ cg)
⊢ cf ≤ max cf cg
case intro.intro.intro.intro.intro.intro.refine'_2
E : Type u_1
inst✝ : NormedAddCommGroup E
a : ℝ
f g : ℂ → E
l : Filter ℂ
this :
  ∀ {c₁ c₂ B₁ B₂ : ℝ},
    c₁ ≤ c₂ →
      0 ≤ B₂ →
        B₁ ≤ B₂ → (fun z => expR (B₁ * Complex.abs z ^ c₁)) =O[cobounded ℂ ⊓ l] fun z => expR (B₂ * Complex.abs z ^ c₂)
cf : ℝ
hcf : cf < a
Bf : ℝ
hOf : f =O[cobounded ℂ ⊓ l] fun z => expR (Bf * Complex.abs z ^ cf)
cg : ℝ
hcg : cg < a
Bg : ℝ
hOg : g =O[cobounded ℂ ⊓ l] fun z => expR (Bg * Complex.abs z ^ cg)
⊢ 0 ≤ max 0 (max Bf Bg)
case intro.intro.intro.intro.intro.intro.refine'_3
E : Type u_1
inst✝ : NormedAddCommGroup E
a : ℝ
f g : ℂ → E
l : Filter ℂ
this :
  ∀ {c₁ c₂ B₁ B₂ : ℝ},
    c₁ ≤ c₂ →
      0 ≤ B₂ →
        B₁ ≤ B₂ → (fun z => expR (B₁ * Complex.abs z ^ c₁)) =O[cobounded ℂ ⊓ l] fun z => expR (B₂ * Complex.abs z ^ c₂)
cf : ℝ
hcf : cf < a
Bf : ℝ
hOf : f =O[cobounded ℂ ⊓ l] fun z => expR (Bf * Complex.abs z ^ cf)
cg : ℝ
hcg : cg < a
Bg : ℝ
hOg : g =O[cobounded ℂ ⊓ l] fun z => expR (Bg * Complex.abs z ^ cg)
⊢ Bf ≤ max 0 (max Bf Bg)
case intro.intro.intro.intro.intro.intro.refine'_4
E : Type u_1
inst✝ : NormedAddCommGroup E
a : ℝ
f g : ℂ → E
l : Filter ℂ
this :
  ∀ {c₁ c₂ B₁ B₂ : ℝ},
    c₁ ≤ c₂ →
      0 ≤ B₂ →
        B₁ ≤ B₂ → (fun z => expR (B₁ * Complex.abs z ^ c₁)) =O[cobounded ℂ ⊓ l] fun z => expR (B₂ * Complex.abs z ^ c₂)
cf : ℝ
hcf : cf < a
Bf : ℝ
hOf : f =O[cobounded ℂ ⊓ l] fun z => expR (Bf * Complex.abs z ^ cf)
cg : ℝ
hcg : cg < a
Bg : ℝ
hOg : g =O[cobounded ℂ ⊓ l] fun z => expR (Bg * Complex.abs z ^ cg)
⊢ cg ≤ max cf cg
case intro.intro.intro.intro.intro.intro.refine'_5
E : Type u_1
inst✝ : NormedAddCommGroup E
a : ℝ
f g : ℂ → E
l : Filter ℂ
this :
  ∀ {c₁ c₂ B₁ B₂ : ℝ},
    c₁ ≤ c₂ →
      0 ≤ B₂ →
        B₁ ≤ B₂ → (fun z => expR (B₁ * Complex.abs z ^ c₁)) =O[cobounded ℂ ⊓ l] fun z => expR (B₂ * Complex.abs z ^ c₂)
cf : ℝ
hcf : cf < a
Bf : ℝ
hOf : f =O[cobounded ℂ ⊓ l] fun z => expR (Bf * Complex.abs z ^ cf)
cg : ℝ
hcg : cg < a
Bg : ℝ
hOg : g =O[cobounded ℂ ⊓ l] fun z => expR (Bg * Complex.abs z ^ cg)
⊢ 0 ≤ max 0 (max Bf Bg)
case intro.intro.intro.intro.intro.intro.refine'_6
E : Type u_1
inst✝ : NormedAddCommGroup E
a : ℝ
f g : ℂ → E
l : Filter ℂ
this :
  ∀ {c₁ c₂ B₁ B₂ : ℝ},
    c₁ ≤ c₂ →
      0 ≤ B₂ →
        B₁ ≤ B₂ → (fun z => expR (B₁ * Complex.abs z ^ c₁)) =O[cobounded ℂ ⊓ l] fun z => expR (B₂ * Complex.abs z ^ c₂)
cf : ℝ
hcf : cf < a
Bf : ℝ
hOf : f =O[cobounded ℂ ⊓ l] fun z => expR (Bf * Complex.abs z ^ cf)
cg : ℝ
hcg : cg < a
Bg : ℝ
hOg : g =O[cobounded ℂ ⊓ l] fun z => expR (Bg * Complex.abs z ^ cg)
⊢ Bg ≤ max 0 (max Bf Bg)

Next tactic:
exacts [le_max_left _ _, le_max_left _ _, (le_max_left _ _).trans (le_max_right _ _),
    le_max_right _ _, le_max_left _ _, (le_max_right _ _).trans (le_max_right _ _)]