[Spoilers] Black Pits final battle and beholder

First about beholder.
Isn't there supposed to be a fight with beholder? Conversation with him seems to mean so.
Instead after 14th battle (with fire elementals) there is only Baeloth fight.


Second: final battle. ( I've found one solution on my own eventually but it would be very strange if its the only one).

I reached final battle in Black Pits. I'm fighting Baeloth. I don't have much problems with Baeloth but i can't find a way to hurt rakshasa Elan Garaq who is helping him.
From what i know rakshasas are immune to spells below level 8 and my maximum spell level is 5. I was trying "Lower Resistance" but it seems to not work at all.
From what could see the best weapon available in shops is one with bonus +2 and it doesnt work on rakshasa.
I made weapon +3 with spell Enchanted Weapon but it doesn't work either.

Lucky me i have priest of Helm in my party and he has speciall spell Seeking Sword that makes sword +4. This eventually worked and killed rakshasa.

I think it can't be the only solution so please tell me how you managed to kill rakshasa.