Hickory: "What's the largest amount of enemies you have killed
with one spell/action?"
The actual killing comes with a single action: telling the Wish genie to cast Horrid Wilting on everything on the map. The rest was just setup to make there *be* tons of enemies on the map to kill in the first place, an action to keep the game from ending (otherwise, you wouldn't be able to see the messages because the game doesn't let you view the combat log after a game over) and a save just in case of bad luck..
Using that Wish effect an any map with a decent number of weak creature should get you a decent number of kills. (Remember that you can export a main character who knows that spell, then import that character into a new BG2 game.)
(Also, I consider casting Wish and actually making the wish to be part of the same action; same with Limited Wish.)
One more thing: Please don't mischaracterize my post when quoting it: My post was certainly not drivel.