Healer classes....in every game, should be that way.
Healers should be able to heal themselves and not die, untill they run out of power(which if you aren't a horrible player, we eventually do), or you team up on them, in which case they will not be able to keep up and will probably fall.
Also, along with their power eventually dying out, yes they can sustain themselves, but typically as a healer(at least for me) although I can stay up for a prolonged amount of time, I(the healer) would probably not be able to simultaneously fight you off offensively without depleting my power.
When they start to fight back and heal at the same time...it really just comes back to who's hitting harder and whos power and utilities will run out faster.
wanna talk about unkillable? bounty hunters =.=