Since he is a demon, wielding a blessed silver saber is very effective. Since he is Large, a dwarvish mattock is also effective. Since he doesn't wield a weapon, applying your bullwhip won't do any good.
Not only is he surrounded by various monsters in his temple and capable of summoning more, but he also does the "teleport away" defense, like demon lords in Gehennom and other Quest nemeses. If you can go toe-to-toe with him (which, by level 14, you should have decent armor AC -15 and one of the weapons listed above) you should engage him, then go stand on the up staircase. Elbereth may also come in handy. Blocking the stairway with a boulder is effective. A scroll of Earth can be handy.
A user has suggested improving this page or section as follows:
"More information about fighting, etc. is needed."
Throw stuff at him from the other side of the bolder. When you stop getting the hit/miss message, quickly run into the small room behind the alter. When he teleports next to you and summons help, there are fewer nasties because the room is small, and your pet(s) should be close by. You should already have a unicorn horn for fixing problems and a cursed scroll of teleport, so you can escape if things go badly. #twoweapon with silver saber + silver dagger helps kill him fast.