[Micah]

If you've got some allies you should go for FA without exception, it's the best way to avoid completely gimping yourself, since it doesn't require you to pull your mages out into the field to spam artillery spells, just to recruit a lot of crossbows, which you'll be doing anyway. Especially if your allies can supply some wine bags to keep them fed. Large armies can be a problem if you're under-supplied, of course.

If you're on your own I'd probably go for evo first in terms of research, as you'll have a hard time feeding your legions of crossbows into Ermor's territory with no native N. Actually, if you're on your own I'd probably just give up, as Ermor is pretty damn unbalanced. Anyhow...buffed holy pyre can also hit stuff in the back of Ermor's horde, as I found out recently, so you might be able to pop a few leaders with it.

Honestly, on a smallish map in a duel I'd probably just spam high inquisitors every turn and get some indie priests to boot if you've got any extra cash and rush Ermor behind some chaff, research be damned. Time is working against you in a big way, as Thaum-5 gives Ermor Burden of Time, which will kill off your entire mage corps in short order, and Enchant-6 gives them Rigor Mortis, which is pretty much game over. Plus their troop generation is exponential. While they're getting stronger each turn you're having your upkeep rise steadily and get confronted with the fact that their dead zone is steadily growing along with the size of the army you'll need to have a shot to enter it, steadily compounding your supply problems. They're also pulling in a ton of gems off the bat, while you likely won't have the resources to keep up between searching, researching, and trying to fend off the ridiculous nation on your border. The ultimate anti-Ermor spell for Marignon is undoubtedly fire storm, and the matrix idea here would work wonderfully, since you should have enough gems together at that point to forge 4 of them. F3 + 1 level from communion + phoenix power will do you. Toss a couple of fire fends on your men-at-arms if you don't have anything better and watch Ermor burn. The problem here is that Ermor can probably drop BoT by the time you get it, and then you have no mages anymore, which is a problem when you're trying to cast spells.

Construction seems like a bad choice, since I have no idea how you'd be able to afford the kind of hardware that's being discussed if you ran straight for construction, especially with default LA site settings. As a research choice down the line though it's certainly a valuable addition to your arsenal.

Giving a priest a crystal shield with 8! spellcasting encumbrance is generally not a good plan. Crystal shields are best used to bump a mage dropping a big enchantment, not one that's casting 0-fatigue spells every round. I can't speak to the communion idea, but if it lets you spam boosted banishes for free using the last caster in the order as the communion master that's pretty badass. I'm still at a loss as to how you'd afford that many matrices, but still, badass if you can manage it. Flambeaus are an excellent use for any extra F gems you have, no doubt about it.

If you've got allies I'd try to get them to forge some of the items discussed while you focus on direct artillery, since you'll be front-lining it, and items can be sent with just a 1-turn delay, so no huge loss.