Reaching him generally proves to be more difficult than fighting him, due to the design of the level. While the Master Assassin can be very annoying, if you're a competent fighter, he doesn't pose much of a threat to you. However, he has a nasty habit of teleporting into the lower left corner of his level (the area that is inaccessible to you while in the larger middle portion) and healing himself. There are a few good ways to take him out quickly and easily:
- Zap him with a wand of death, if you have one, to get rid of him instantly.
- Throw a potion of sickness at him and he'll soon be gone.
- Throw a potion of sleeping or paralysis at him, then beat his head in. You may wish to wield the potion if you want to be sure it will hit. Zapping him with a wand of sleep can also achieve the same ends.
Alternatively, after he lands on the upstairs, leave the level via whatever means you have prepared and re-enter. This will remove the whole "hard to reach" part of the conflict and reduce it to a basic boss fight.
Instead of reaching him directly, you can also use a drum of earthquake, a cursed potion of invisibility, the shriek of a shrieker, or the psychic blast of a mind flayer to wake him up while you're standing on the upstair. This will cause him to warp to you and attack, while preventing him from escaping to the floor above.