'Tis true, but only cause Sarevok has an ass-kicking 100% magic res or something like that. It's hard to land a lot of spells on him anyway, and I can only say Thanks Angelo for getting your dispell magic off on him so he's not running like some overpowered train any more.
And besides, he was powerful. 5 mm shows that he was lvl9 and above, and that was pretty nifty for bg1. He was also able to release them very fast with some trick, so he was probably stronger than any mage you were able to build in that game.
But well, getting toe-to-toe with Sarevok, being a mage, in the final battle was never especially good. Especially not if you're like me, and just running through the game, getting your chars to lvl4-5 and getting two-hitted from the big bastard. ^^
And, don't forget that a whole party of ~lvl10 chars, you yourself didn't stand a chance against Gorion.