Falling frost vs. AI hordes

[Meursy]

Nope, flying in the same phase as regular movement.

Instead of just trying to head him off, and playing a purely defensive guessing game, and gradually lose provinces (which I've done against the AI, and I don't know if it's your case), consider letting one of his armies wander into your territory, and move your intercepting force(s), the ones you hope to catch him with, in such a way that they either get him or force him into a dead end. And always try to take the province behind him each time he moves (i.e. the one he's standing in each turn). As long as you keep doing that, you'll catch him eventually, and when you do he won't have anywhere to rout to, so 100 percent kill rate.

Or, send your armies off into the enemies territory (or again, always attack the province he's moving from each turn), and instead try to place some artillery mages in each province he can move to, and buff the PD a bit, but not too much (Marignon has decent PD, right?). The idea there is to whittle him down, and have your mages flee when you PD gets chewed up.

Of course, this sort of stuff is effective against a human player with comparable resources, but maybe not an artificially boosted AI. I think the real way to beat that is just to make some SC it can't counter, or research the uber spells it can't use effectively.