I decided to do a spreadsheet to figure out the numbers and then discovered Excel just straight up has a function that does it for me. Hello BINOMDIST, thank you very much. Here are the odds against any number of 9AV targets (columns) of getting any number of KO+ removals (rows).
So I was 55% to remove 0 orcs when I shot a fireball at 8 of them. Maybe this play would make sense if I was desperate to get lucky, but I didn't need that at all. I'm convinced the attrition fireball was a mistake, and I definitely got baited.
On the other hand, if 7AV elves were to make the same offer we'd get the following:
Now we're 75% to get rid of at least one and have pretty good chances of removing multiples. They're also better at recovering from the lightning bolt, so if an elf team were to give me that opportunity I would jump all over it.