Are you not interrupting Ironfist?
If you interrupt headshot that fight is very, very easy. If you do not, then yeah it gets to be pretty hard.
I just did Irofist with a newish level 50 Vanguard tank. Ironfist didn't even get to headshot. He hit the tank with his insta abilities for 9K and then 6K within about 2 seconds. He never stood a chance.