I've only done him 3 times, first time took about 4 attempts and seems totally random, second and third times he went down first try and tbh I don't think I did anything different other than me not killing the healer

I'd definately suggest having 2 healers for the fight.
I've no idea what other classes do when they're MC'd but I mainly shoot or gouge as well as use all my cooldowns so it's easier for me to just burn them all right away.
I do tend to not bother running back to the fight after he does a knock back though as he usually MCs just as I manage to get back, I'd much rather spend the time I'm MCd running towards people rather than hitting or being hit by them.