Falling frost vs. AI hordes

[krpeters]

So, I was tinkering around in SP yet again, and I've discovered that Falling Frost is a reasonably effective spell against the hordes of garbage troops the AI likes to throw at me. The area effect seems to be the critical factor. Hitting five squares at once is *very* nice, for a low-fatigue spell.

What I'm wondering is, what are some other low-cost options for wiping out the piles of troops more difficult AIs attack with? I'm not looking to merely scare them off -- a 200 troop army that you scare off just adds to the 300 troop army behind it creating a 500 troop steamroller. I've been underwhelmed by thunderstrike and blade wind (the latter is nice against shieldless troops, but otherwise worthless). Falling fires is mediocre. Astral Gyser is great against low-MR troops but ineffective against decent troops.

What else should I be looking at, without needing to research Evoc-9, if I want to incinerate AI troops with my mages?