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Every Medic possesses a 40/150 Syringe Gun (Primary), the class defining Medi-Gun (Secondary) and their last resort, the Bonesaw (Melee). They may also unlock a series of alternate weapons by successfully completing Medic Achievements. Use the Bonesaw sparingly, at point-blank range. Melee kills are always fun, and melee weapons have taken quite a boost in power since the Team Fortress Classic (TFC) crowbar, but don't get cocky. Save your Bonesaw for stealthy strikes from behind, or if your attacker is right next to you and retreating isn't an option. Otherwise, you're usually better off healing another teammate and letting them finish the job for you instead. Use the Syringe Gun for distance shots. As the name implies, the Syringe Gun fires hypodermic needles. It is not the most powerful weapon in TF2, but it is often underestimated by most players. An experienced player can be lethal with the Syringe Gun in their hands. Note that the needles from the Syringe Gun actually fire slightly to the right of where the crosshair is. Fire at a wall from varying distances and look where the needles land to get a feel of where to aim. They also arc downward rather quickly. You'll need to aim upward and to the left with the Syringe Gun if you intend to hit any enemies from a distance. This does, however, make it useful to shoot over short hills. The Syringe Gun's needles travel through the air slowly, which can make them easy for other players to dodge them. Keep this in mind. Try using the Syringe Gun to provide cover fire for your team. They'll stay in the air long enough to travel at enemies that like to sidestep back and forth from corners. Since the Syringe Gun's projectiles move through the air slowly, it is a good strategy to fire if an enemy is chasing you, with you either standing still, moving towards him, or backtracking. If he moves towards your syringes, he is more likely to get hit and thus die. This is essentially a given; your teammates will respect you for being able to heal them effectively and quickly. The biggest change to the Medic since TFC is his ability to heal from a distance. You can now space yourself out from between your teammate as you heal them to avoid splash damage. You don't have to keep your crosshairs on the player to keep healing them, so you should look around frequently to keep an eye on the rest of the battlefield; it's a good idea to look for other wounded players, and keep an eye out for approaching Spies and Pyros. In addition to healing, your Medi Gun can raise a player's health to 150% more of their maximum health. This will slowly return back to its maximum when you stop healing them. You can tell how "overhealed" a player is by the white glowing/pulsating surrounding his or her health cross; the bigger this glow, the more over healing they have received. You will earn an "assist kill" if you are healing a teammate that gets a kill point. As you heal players, you will notice that the small bar on the bottom right corner of your HUD will start to fill itself. This is your UberCharge meter. When this reaches 100%, the Medic will announce that he is "charged". By right-clicking, you will activate the UberCharge, which will make you and your healing target completely invulnerable for a maximum of 8 seconds Note that you and your target cannot capture points when Uber Charged. Similarly, you and your target will not be able to carry the enemy intelligence (flag). However, you can still prevent the enemy team from taking a capture point while UberCharged. Your UberCharge meter will fill fastest when healing a damaged player. It fills slowest when over healing a player who is already at 150% of their maximum health. This gives Medics an incentive to take care of his team as a whole, rather than to attach themselves to the nearest Heavy. The announcement of "I am charged!" can be heard by nearby enemies. If you wish to cover up this tell-tale signal so they are less likely to be prepared for it, you can call for a Medic (or any other vocal command) just as the meter reaches 100%, and it will override the passive "I am charged!" announcement.
Get to know his weapons. Use the Medi Gun to heal your teammates. Use your UberCharge to make you and one of your teammates invincible for a short time.