Yeah paladin and warrior have absolutely 0 issue staying alive in mob pulls, which mean they have 0 issue staying alive in bosses. Gunbreaker's a little harder to self heal through mob pulls, but with a bit of luck and good enough dps, it's doable. When it comes to staying alive in bosses, again, gunbreaker's a little harder to do so... but only a little. I remember playing gunbreaker in the grand cosmos, final boss. The healer and both dps die and the boss still has a good chunk of health left, probably about 40 or 50%. It was difficult, it was tedious, but I brought the boss all the way down and kept myself decently healthy the entire time. So dark knight's definitely the least survivable tank but I'm sure with the right people it, too, could replicate what the other 3 can do.