AT should focus on the rear or the treads. There is a 1-shot kill spot on a tank where the turret meets the hull. But the angle has to be right as the result is a hit to the hull and a richochet to the turret. It's next to impossible, so don't focus on it. Instead, go to the roof tops of nearby buildings as the guns on the tank can't aim up to that elevation; fire down onto the turret from a safe spot

Sniper: .50 cal is great at camping chopper spawn points. Line up perfectly for a head shot and the just wait. Make sure you get the guy the minute he gets in the seat, otherwise he may start to rotate the bird and you'll lose the shot.

Supply: short bursts! Prone!

Engineer: mine all bridges!

Supply and Medic should throw out bags every chance they get, even if a friendly isn't nearby.... someone eventually will come that way and you'll get the point.