BG1 Sarevok is overpowered into the sky.
Its a Level 17 Fighter with amazing stats and uber gear, plus some followers who are not as extreme, but also very powerful.
While your group is like, what, level 8 to 10, depending upon class, even less for multiclass.
The only way to win is cheese. Cheesing the AI, or cheesing wand summons (needs an original version of BG1, without the five summon limit).
The low hitpoints of F/M/T doesnt matter though; Sarevok cannot get to attack your character anyway.