Yeah, I've killed him in the original game, anyway. As soon as he turns neutral, you pause and set up a fiery barrage to deal as much damage as possible. Think they made it impossible to kill him there in BGEE.
I don't usually bother with backstabbing unless it's the main character doing it, like with my old Elf Stalker. I've always just had Imoen, Yoshimo, etc sit back and launch arrows, unless the circumstances warrant fighting with a sword. Swashbuckler Imoen specialized in short bows is a pretty reliable shot.