My preferred class is the infiltrator, been using it since the first game of Mass Effect.
Now I know this is generally what people go for anyways when they are playing games. Its the way you go about it, you have to show patience and a strong will to live.
Playing as an infiltrator in Mass Effect 2 you get the cloaking ability, and despite the fact that when in use it doesn't allow you to regenerate all you have to do is get to a safe location and by then you will be out of combat long enough so that just a few short seconds after the cloak wheres off you will regenerate back to full health.
I've gone through the entire campaign with this simple guerrilla tactics of hitting, running and recovering. Because while playing the enemy will continue to shoot at you and while in combat you are always in danger and given enough time in combat you will die.
So you delay this by exiting this repeated sequence of death to a near death experience by avoiding that final bullet that's supposed to kill you.
Going into a fight you examine the area and where the next cover would be that you could quickly cloak to, then picking as many off as you can with a sniper rifle you kill the major threats or anyone who has an angle on your squad-mates preferably the latter choice.
If you come close to death or a larger foe is charging you, you quickly cloak and run in the opposite direction and line up another shot since you have a cloaking damage bonus then del as much damage before you become surrounded.
Eventually he will die and you would have almost died a few times, but you survived and you got the kill regardless of how long it might have taken or how close you came to starting over. Survival for the kill.