gorecrusher the behemoth
While fighting gorecrusher, being right in front of it, they will continue to disengage and end the fight refilling their health bar. I have fought them in their lair and just outside of it on the path and they still run away and refill their health.
edit: I first started the fight right after it respawned. After I let it run away for a couple minutes and then fought in its lair the fight went normally. Perhaps its a bug after it respawns that it has to go back to the lair first?