Yeah, just to clear a couple things up first, Gunslingers can not heal, nor can they stealth. That is a Scoundrel you're thinking of.
Second, I play a Scoundrel, yes a squishy Scoundrel with thin armor and precious few defenses. I don't play any force user classes higher than level 15 or so, and I say Marauders and Sentinels are fine. Do they do massive damage? Hell yeah, but it seems to me that's what they are supposed to. They are the only class I really worry about in close quarters. When they are not around, I can pretty much rip people apart at close range with impunity. However, when Marauders/Sentinels show up, I really need to be on my guard, as they can rip to me shreds in seconds. I call that pretty balanced, in my opinion.
Like another said before, get a good ranged class focused on them, like a Gunslinger, and they go down pretty quick. All classes have their strengths and weaknesses, you just have to hope your team has the proper class composition and knowledge to counter the enemy, it's that simple.