Do you mean Ba'al, in the Broken Valley mine?
Are you having trouble doing damage to him, or with the damage he is doing to you? See
here if he has an over-powered attack.
Lowering the difficulty may help.
In D2:ED as a warrior I rushed in right away and went around the outside of the chamber taking out all the minions first, then attacked Ba'al.
As a ranger in DKS I found it best to stay in the hallway and draw the minions to me, even though Ba'al could hit me there occasionally. Having the creature summoned right away wasn't a net help, as it drew opponents into the fight faster than if I targeted them myself.