Marauder's are designed to be glass cannons in the first place pack punch but can't take much of a beating. That is why you need to use either a tank companion or healer. Also why they have some of the best defensive cool downs in the game. The trick with this class is make sure you are interrupting enemies so they can't hit you with their biggest abilities.
Excellent advice. Also helps to burn weaker mobs first. They do a surprising amount of damage if left alone.