Having never done a heavy armor character, all I can say is that you gotta bail as soon as it prepares an attack. I would usually sprint away, then sprint up to a foreleg and mash away at his head. Its dash->body slam attack hits like a truck (if not kills instantly?), but iirc its path swerves very little, so you can sprint off diagonally in the direction it goes, then wait to recover stamina, then go back and attack for a few seconds. I recall the dash and stomping attacks having an annoyingly misleading range and then applying stagger/knockdown.
edit: Oh wait, I'm remembering more about this fight. I think the head takes much more damage, and if you stay near its head, it'll try to body slam, which has a longer recovery time for it. IIRC, you can bait it into doing that attack a few times in a row to get some good hits at its head.
It's honestly the most disappointing fight in the game, considering the trailer's reveal of was so damned magnificent.