From what i know about shandalar is that he is the hardest killable NPC (even worst than Drizzt). This is the only NPC i failed to defeat in the whole game, from the fact that i can kill drizzt by using tactics yet there is nothing i can do to Shandalar show that he is "virtually" indestructable.
Of coz if MMs can really hurt him, then killing him would be a piece of cake.
btw, y did they want him to be killable? It make no sense giving him -20 AC and 100% resistances to all elememts etc. Going by the AD&D rules no mortal mages can reached this stage no matter what. Of coz giving a 26000 xp is gorgeous in BG/ToSC but there is really no point for doing that.