i just stunned him while he was in the stupid 400% damage dealingpart, and used my strong attacks when he was at the 400% more damage taken part. my attacks were hitting for 5000-8000 during those parts and he went down pretty easy
Try to defend yourself or stun him while he has the 400% damage thing on. Then when he is "reloading", set your allies to relentless mode (unless they are on the brink of death, in which case you should let vera do some healing first). Attack him with all you got when he is reloading, it shouldn't be too hard.
