I always thought the best solution would be:
1) Monster dies
2) Check the area for another target that would have been hit by the AOE
3) Found one? Continue spell (centered around wherever the original target died). Nothing there? Cancel.
It is soooooooooooooooo frustrating to have your AOE targetted on one of the few monster that die where then there still several others crawling around available to be hit. As many have stated, the Cutter's Cry mid-boss (Myrmidon Princess) and her many adds is the perfect example of this.