[Squirrelloid]

So, from the sounds of things, he's moving around with a single big army, and camping somewhere occasionally to build forts? Lol.

Ok, its obvious that a head-on fight is unwinnable. So don't fight his army. At all. Avoid it. Take all his other provinces from him. Push for his capital and siege it. Let him chase you around the map. Push a research advantage fueled by the gold advantage this gives you.

If he sieges one of your forts, so what? Your maenad-generating pans make your forts unbreakable - let him sit out there while you keep researching. He's just wasting his time with his army. And of course your other forts are still producing mages and armies of maenads to go plunder his lands while his army is occupied.

At some point he's forced to either break up his mega-army or hemorrhage cash and thus troops like mad as his income plummets.

Basically, when confronted with a large monolithic army, raid the hell out of him.

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Eventually you research some useful remote attack spells. Spells which single out commanders can help break his army up, and spells which attack his troops can whittle down his numbers. The best for you is probably Beckoning because its a nature spell (in thaumaturgy) for which you'll have plenty of gems and casters, and its on the way to Charm, which you'll want anyway.

Also, use summons. You should be able to Dragonmaster some Pan and mass Cave Drakes (although this is harder in vanilla because dragonmaster is punitively hard to research), which should absorb archer fire reasonably well. Ivy King thugs, troll kings with retinue, kithraonic lions, fall bears, and eventually Faerie Queens can probably all find a role in your armies, with paths you have easy access to.

I'd strongly consider breaking into one of death or blood (if your a Pan era that has plenty of blood access, do that) to add some variety to your options.

Tactical: Use archer decoys! Put some troops with shields up front with orders to guard a commander in the back, watch his archers run into your hordes of screaming women while trying to fire at the withdrawing troops.

When you manage Faerie Queens, make arrow fend routine.