I am playing on heroic, was going well until I hit the excavation site. I think there is a bug with that boss. My first try I didn't get a cut scene and he had no health bars. I could see I was getting his shield down, but couldn't seem to kill him. After quitting out of frustration (died 20x times) I tried again the next day. Got a cut scene and saw health bars. Took 2 tries to get him down, though granted I overprepared for that attempt. This boss is notorious as the worst boss in the game. If anyone has trouble with him, there is an easy way to kill him see spoiler:
you can cheese him by grappling to the ledge above the doorways from the main room, he will sort of bug out as he can't reach you with his hammer
