Yeah, it can be REAL hard to stay alive. But if you are the engineer class, once you start getting shot at, run away. Repair your vehicle, then go and fight some more. It REALLY annoys then enemy when you do that, especially with the tanks, and the walker (forget its name)... STOMP..
Also ALWAYS shoot flares out to divert heat seekers. When they lock on.. FLARE AND RUUUUNNN!!!
The hog is a great tank killer, but you have to DANCE up to a tank or it will blow you away.
You can hide around corners near an enemy spawn point. When they respawn and are running to the objective, with their backs to you, they become... BOOM.. Nothing but speed bumps.