[Agema]

Surely it's still all in their best interests to flatten the guy with AN up?

It can depend on what magic you have available beyond your basic mages, who as I recall are death/nature with some astral and water. Fog Warriors and Earthquake/Rain of Stones may not be viable, nor big artillery like Murdering Winter or raining fire. If he's got AN up, quite likely he can also cast Army of Gold/Lead, which blunts Earthquake/Rain of Stones a lot.

Skeleton spam might be good - GfH is high fatigue and inaccurate, so it might drain his fatigue lots whilst you don't take much damage. Your undead are immune to cold and poison, so foul vapours to poison his army and Grip of Winter to fatigue them - particularly powerful in a long battle where you're throwing spam - although you'd want to protect any of your mages from posion/cold as well.

As a desperation tactic, you could always create a load of S1 mages and try to kamikaze arcane duel his communioned mages to death.