Baldur's Gate I, for those who are not familiar.
It was quite silly, yes, because even if you prepared Imoen beforehand to backstab and shot arrows at him like crazy, with the THAC0 of a level 1 or2 character it was almost impossible to disrupt that mage before the Mirror Image went off. The only way to get him easily was to lure him to the guards then run away.
I never had that much trouble with the assassin. I mean it took a few attempts but it wasn't anything that made me curse the game.