Forgot you probably have a party, so here is a strat.
Again, set traps in the room before if you have a thief. This will be the fight zone.
The magic types can cast the beneficial spells now or later if you have the boots of speed or several potions.
With boots/potions one fighter type goes in and whacks him and starts the ball rolling. Start running around the room like a scared girl(or solo assassin
) when his haste runs out, let the MU's start casting the beneficial spells and then lure him into that room.
The traps should do some damage, but not kill him. Let your magic users and ranged attackers line up around the room hitting him with arrows and Melf's Minute Meterors of magic arrows if you have them.
Get all your tanks around him and just keep beating him, and let the ranged guys do their work. Don't be afraid to use every spell you have as you can rest afterwards.