Believe it or not, stabbing the bastard in the face with your knife by pressing R3 seems
to be the most effective way that I found. This keeps him from firing his weapon at you.
You can mix shotgun blasts with your stabbing to speed his death along. I just beat him
on hard using this method.
The only trick is to get close enough to stab him in the first place...

first of all you have to sort of win the fight upstais (you dont kill him it just moves
back down stairs)now while downstais you will notice that the passage that you came into
the dance are is available and it is full of guns, get the heavy machine gun and blast him
,dodge and blast dodge and blast,it takes much less time than the shotgun

I beat him 10 times by now, but it was easy at first. Dont try to use a scope or crouch
at first. when you first see him start shooting moving backwards. If he shoots you and
you start turning red just move out of his site, then when your color returns do it again
and again. once he moves down stairs its easy. break out your scope and crouch on the
stairs he wont come at you on the stairs. Then just pick him off till he dies.