How to play TFC.
Step 1, go HW Guy.
Step 2, if you see a bad guy, shoot at him with the Assault Cannon.
Step 3, throw a bunch of MIRV grenades while you're at it. In the unlikely event you're out of them (as you have a fuck-ton) spam regular ones.
You now have the ablity to kill anything standing in a huge cone in front of you, even if they're behind a wall or some other form of cover. Also works with Demoman!
Failing this, use the alternative route.
Step 1, go medic.
Step 2, throw a concussion grenade at the ground and jump over it.
No step 3, that's it. You are now a class with excellent health an armor, two of the most versatile and effective close-range weapons and health regeneration. Also yeah you can heal people, but it's not worth it. You're primary role is to be significantly more effective than the other 8 classes at completing all the other objectives. Healing people is very low on your priorities list.