Improved Ilyich - Tactics Remix

can you please give advice on how to deal with him and his gang?
mod: Tactics Remix + Forgotten Armament
component: Improved Ilyich
my custom party:
3x fighter/thieves
priest of lathander
priest of tempus
skald
mod: Tactics Remix + Forgotten Armament
component: Improved Ilyich
my custom party:
3x fighter/thieves
priest of lathander
priest of tempus
skald
0
Comments
author states that nightmare is recommended for 500k exp characters.
mine is 89k exp.
insane + double dmg
i defeated them right in their room, in direct, fair fight, no tricks. i did it several times to make sure i did not get lucky. won ~80% of the time.