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! Zap him with a wand of death to get rid of him instantly. It's entirely likely that you won't have a wand of death at this point, so:
Gas Him! You know that potion of sickness you start the game with? Throw it at him and he'll soon be gone.
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.