The way i see it. The "good" dr was able to beat pinhead so easly because it was not only the dr's will to kill Kersty was not only his but Leviathan 's as well. Since they where one at the time. By pinhead opossing the dr. He was also opposing Hell's order.
This time around the dr doesnt have Leviathan attached. So he isnt nearly as powerful. Both have the respect of Leviathan on their side. Both spent their lives studdying "Hell" so have a master of its do's and don't s . It would be an intrestinf fight to see how powerful the dr will be without Leviathan's aid