Woo, someone helping with creating content in this forum!
I don't know exactly what I should post here, but I have most experience with soldier, so here goes...
1. Combat Medicine is your friend, as tempting as the other skills are, try and max this first.
2. Other skills worth a point in are 6th sense (more than lvl 1 is a bit of a waste) and burning bullets, though blasting strike and grenade spam are handy too.
3. If being chased by a knifing commando, do not just run backwards. Try and wheel around and confuse him, or blast him away.
4. [All Classes] Try not to stay still when capturing flags. Yes, crouching may hide you behind a low wall, but beware of invisible knifers approaching your position
That's all for now folks!