How the hell do you fight remake Salazar

He constantly follows me and does unavoidable attacks, like chasing me and starting his vomit attack next to me while I have no cover.

I try to go up the stairs because he's behind me? he goes to the ceiling and does a vomit attack while I'm crossing the ceiling.

It genuinely seems only possible to avoid these attacks consistently if he does them while you're already at the stairs since he can't be stunned during them.