put a time stop scroll in one of your quick use slots. cast simalicrum, i think thats what it was, then you have a second version of "you" also with a time stop scroll in their quick use slot.
i did this in the final fight, by accident really. I ran out of offensive spells with the fake me, and ended up casting invisibility a few times to backstab Irenicus. I was a mage/theif.
i won't say this is the best way, but it was an interesting surprise at the moment. I wasn't high enough level of a mage to memorize the spells, so i just had a bunch of scrolls lying around.
backstabbing is a great killing method. also, i like a high level mage casting Chromatic Orb. Another thing, i havn't done in BG but rather in the original Pool of Radiance, was turning undead to destroy them. I'm playing a preist this game, so i look forward to the chance
dulouz.