Master Kaen

Master Kaen is the monk Quest nemesis and considered one of the most difficult nemesis.

Boulder fort
After Kaen ports to the upstairs, go up and read a scroll of earth and use ranged attacks such as spells or wands. You can also try this on the upstair to prevent him from porting away.

Drain life
Use the drain life spell until he croaks. It is a good idea to do this on the level above and when he teleports away, go downstairs to heal, preferably on a burned Elbereth.

Levitation
Levitate or be fire resistant with fireproof water walking boots over the lava and use ranged attacks. At a distance, Kaen only uses the heal self and create insects spells.

Melee
Fight Kaen mano-a-mano. Not recommended.

Paralysis or sleep
Wield or throw a potion of paralysis while wearing a ring of free action or zap him with the sleep spell. Make sure you can hit him multiple times to keep him immobile.