1- From sleep helpfile:
A victim who resists this spell will be briefly more wary against a second
attempt.

So, if you failed your first atempt of sleep, i recomend you to run around and misguide him for about 2 tics to try again. If you think you have time, try to land a curse between your atempts (lower his spell resistance).

2- At this level, you can get stronger zombies. Use always the strongest zombies you can.

3- Order all rescue and blind are your friends.

4- Summon him away from his servitors and use some kind of lag, let your minions do the dirty work. (If nothing else, you have lash).

5- And last one from me, if he is still doing too much damage, think on using more preparation ... stone skin at least, there are many of those around.

Have fun.