KiNgBrAdLeY7: I found a cheesy method to kill that damn mage. I needed badly his robes of the neutral archmagi, plus he had some scrolls that the Ulgoth Beard innmaster did not sell, among his selection. I imported 5 copies of my dwarf cleric (maxed). 1 of my elven mage, with improved invisibility. The clerics had 3xAnimate Dead and 2xSummon Animal memorized. I had them simultaneously cast all these summons, getting the entire field crammed full of creatures. Pause, 5xChants cast spread throughout the groups, mage gets improved invisible. Approach until Shandalar is visible, pause again, target/attack him with all summoned creatures selected and dragging mage a bit back (so as to avoid dialogue).

If you are lucky enough, while invisible, you can cast magic missiles to interrupt his cast of that shield thing, or fire that kills 3/4 of your summons at once, without him triggering conversation. Fire missiles, go back. Fire missiles, go back. His casts are disabled now. Mage is near to be visible again (duration ends), so take him back away out of sight. Wait 5-8 minutes for the summons to kill him (1-3 damage at best, most hits are critical misses). Go loot him, export his robes and scrolls; scrolls in case you have another character who utilizes wizard spells, and his were high level, and robes, because they command a high price in the market, making you able to raise a fortune and buy anything you need.

Coelocanth: Seriously, man, why would you go to all the trouble of importing 5 copies of one character, spend time summoning, buff your characters and summons, then spend 5-8 minutes waiting for your summons to take down an NPC that was clearly not intended to be killed before a covo script fires, when you could simply:

CLUAConsole:EnableCheatKeys()

then

CTRL+Y on Shandalar. Dead mage in about 20 seconds.

And all this to loot spells and items to sell for gold?

CLUAConsole:CreateItem("xxxx")
CLUAConsole:AddGold("XXXXX")

Honestly, stop trying to convince yourself that what you're doing is any different than using the console and just save yourself some time.

Ignoramus: If you wish to kill him without cheating, try spamming him with wands of paralyzation, and whack him to death once he fails his saving throw. That's pretty much the only legit way of doing it. Of course, you can always import your level 90 terminator/juggernaut from Baldur's Gate XXVII and eat him.