This is an exploit which happen to me today on my warlock, in Heroic Gundrak.
This is what happened:
Before enraging Gal'darah, I placed the Demonic Circle outside boss chamber.
When we engaged him, and tank died, I ported out to my circle, and started running away from him, hoping he will reset.
But he followed me, and after some kiting (He was in troll form) he eventually killed me.
But here is the deal, he did not reset after he killed us all, but remained on stairs, with the HP he had from our wipe, and doing nothing. He evaded first attack, but the rest landed, and we killed him without him fighting back.
I am unsure if this will work again, but if anyone is going Gundrak, please test it, if it works 100%, this is quite good
And beware, I tried this only once, and I don't know what may happen in your case.