Demogorgon

Demogorgon. If you meet him without being prepared for him, draw up your will and prepare for a bones file. He has four attacks: A spell for 8d6, a sting attack that drains a level, and two claw attacks for 1d6 plus disease. He's even tougher than the endgame Riders. The only way to survive him is to be prepared. Be very fast and unburdened, capable of curing diseases, and be able to attack him. Magic resistance and cancellation go without saying. Generally, you will need speed boots or haste self, and either a unihorn or Staff of Aesculapius for equipment. As for how to actually kill him, there are several ways. First, you can beat him to death, but this is very slow and difficult to do. He is not immune to stoning or disintegration, so you can kill him in that manner. If you can reach the up-stairs first and engrave Elbereth there, you can beat on him freely with no risk of illness. Finally, if you can get away, GET OUT! Leave immediately and get past him. Discretion is the better part of valor.