Though defeating Maugneshaagar is not too difficult, you will want to prepare adequately before confronting him as his melee attacks pack quite a punch. Because Maugneshaagar starts on the opposite end of the level, you may (in the worst case) need to walk over to him, wake him up, and chase him back to the upstairs in order to kill him. If you can't teleport directly back to the upstairs, you will want to be prepared to take several beatings from him while you walk back to the stairs while he teleports back and forth to attack you. Surviving this punishment will require a strong AC (of -20, at least), supplemented by standard defensive tactics designed to give you time to heal, such as scrolls of scare monster, a reliable means of carving Elbereth, or a wand of extra healing.
Maugneshaagar is not particularly sturdy, and a Necromancer with a highly enchanted Serpent's Tongue will find that it does not take many consistent hits to finish him off even with Maugneshaagar's poison resistance. Your reward will be The Great Dagger of Glaurgnaa, a great dagger whose most useful ability is to recharge your mana when invoked.