About final boss's immunities...(Solved, after a fashion)

Emeus

This might looks a bit weird but, I'm soloing with a enchanter, he's level 11 before the final fight, and skip the babblings, thing is, I tired hard, and successfully feeble minded the boss, yet it still act like a free man, hold monster also seems useless as well(strange, I heard it can be webbed, but not held?), so I'm stuck in here, help plz?

Emeus

All right I finally made it... with Caelar's help, just dump all the offensive spells, the secret is the lvl4 spell Teleport Zone! It ignores magic resistant and doesn't allow saving throws. Buff yourself and cast Spell Immunity: Abjuration before entering the boss chamber, then ask Caelar to side with you, and give her anything you could offer, equipment that she could wear, and potions such as oil of speed, potion of power, potion of regenerations, etc.

After the fight begin, first cast Improved Invisibility on Caeler, and order Caeler to use her Shining Bolt or simply attack the boss, after the boss casted dispel magic, order Caeler to buff herself and let her focus on the boss, cast Teleport Zone to scatter enemies, cast Slow and summon monsters to draw their fire, eventually Caeler will finish the boss!