The Overweight is not the best at capping because of his slow speed. He's decent at defending the intel but a Constructor is usually the better choice for that task. He's good at area denial (space control), defending CPs, but his biggest strength is keeping the enemies busy. When half the enemy team is desperately trying to kill him, then they are not defending their intel and not capping your intel. His teammates will have an easier job doing their respective tasks.
Aggressive vs Defensive Behavior Edit
It's generally better to fight defensive than aggressive. You want your enemies to run into your fire and take damage and not vice-versa. However sometimes it's better to be aggressive, e.g. to finish of a weakened enemy or to kill someone carrying your intel. To be aggressive or defensive depends highly on the situation and you will get better at judging it with time.
Infiltrator Checking Edit
The act of Infiltrator checking is pretty easy and fast: Shoot a few bullets to the left and to right. If the air bleeds you found an Infiltrator. You should Infiltrator check often, because it's so fast. You can even do it during a fight. Infiltrator checking does not only reveal Infiltrators but it also scares them away. We all know that gibs can cause lag, so it's best to choose "blood only" in the gib options. Don't disable the blood or else Infiltrator checking gets a lot harder. If you hear the sound of the Revolver or of a Knife, immediately turn around and shoot.
Do it. Often. It will greatly decrease the damage you take. It helps win most Overweight vs Overweight fights. However you shouldn't jump in two situations: When your Healer-Buddy is low on health, because he needs your cover then, or, if you're fighting against an autogun, it will just mess up your aim and the autogun hits you anyway no matter how much you jump.
Fighting without a healer Edit
An Overweight without a Healer is like bread without butter. You can be useful to your team but you won't get far. You are easy prey to Riflemen, Rocketmen and intelligent Detonators.
Only eat them when there are no enemies on the screen. Infiltrator check before you eat the Manvich and when you're done IMMEDIATELY jump to the side and Infiltrator check again. It's good to have a teammate around who Infiltrator checks while you eat. As said earlier it's easier to fight aggressive enemies. But sometimes they won't charge at you, so you should bait them to make them fight more recklessly.
You can bait your enemies by flashing the Healer bubble. They'll think you are low on health and fight more recklessly. This won't work against good enemies, an alternative is to flash. Other methods are eating a Manvich or taunting, though this is not recommended, as it is a bit risky. If you eat a Manvich and you are at full health, a backstab won't kill you but will leave you with one health point. You can use this to reveal bad Infiltrators.
Tactical Retreating Edit
You're not invincible (unless you're supered), so retreat when your health is going down OR when there are too many enemies OR when your ammo/overheat bar is low. Retreating is the most important skill, it's the main difference between good and bad Overweights. When to retreat depends highly on the situation, it's a skill you'll learn and get better at with time. Before you retreat tell your Healer or else he will get exposed to enemy fire. You should try to not die at all, because it takes ages to get back to the front line.
Fighting with a healer Edit
Now we are talking. A good Overweight-Healer combo can be almost undefeatable.
Protect your healer at all costsEdit
The Healer cannot effectively defend himself with his Syringe Gun, and his health is very low. In addition, the Healer must keep his primary focus on healing you. If you're getting attacked from two sides, always kill those behind you. If Firebugs (or Quotes) attack, you must concentrate your fire on them, and try to take them down as fast as possible; they are one of the biggest threats to your Healer. Infiltrator check often. Pay attention to what your Healer is doing. If he stops healing you or if his health is decreasing, there's probably someone attacking from behind, kill him. Don't stand in a spot where the Healer can't hide behind you.
Your Ammo/Overheat bar Edit
Always keep an eye on your Overheat bar. If you let your gun overheat, you won't be able to fire for ~1 second. As I said earlier it's best to retreat when the bar is low.
Sparse fire modeEdit
You do this by repeatedly clicking the fire-button. If you shoot this way the overheat will neither increase or decrease. You do roughly half the damage you do otherwise. This is useful for Infiltrator checking and area denial. This is also useful to avoid overheating your gun when the bar is almost down.
Fighting against certain enemies Edit
Keep moving away from them, aim well and keep jumping
Take another path or wait for the Rocketman to come closer or wait for him to run out of rockets.
The one who jumps more and is more defensive will usually win. Good aim helps too.
Just remember that you can always retreat and let your Healer heal you. You can also attack from above.
Keep in mind that Detonators can hit your Healer even while your Healer hides behind you.
Shoot at them until they die.