[13lackGu4rd]

you're MA Caelum, which means you got a ton of air mages. befriend whichever lightning god is your favorite, I'd go for Zeus myself, and use it! no monster can be made immune to lightning from scratch IIRC. max resistance from spells is 50% and they don't stack, so the only way would be Storm Warriors(or similar spell) combined with A9 bless, which of course requires an A9 bless as well as the troops to be sacred. now, with that in mind, you don't need to spend many gems at all. you need 1 staff of storms, a few rings of lightning and a few gems to cast Wrathful skies. bring a few mages with you, as many as you can afford to make lightning immune, equip them all with the lightning immune equipment, and give 1 a staff of storms. turn 1, summon storm power for everyone, turn 2 Thunderstrike for your A3+ mages(so 4+ with summon storm power) other spells for the others. just make sure you got 1 mage to cast Wrathful skies first, before Thunderstrike spamming. you need your own lightning immune troops as blockers, so they won't suffer from the Wrathful Skies and the inevitable collateral damage from the Thunderstrikes. I'd certainly mix in a few thugs as blockers against large AI armies, so even if your regular troops fall too quickly, your thugs will continue to hold the lines long enough for the combination of Thunderstrikes, Wrathful Skies, etc to finish off the entire AI army.

another option, which is much cheaper and easier, is to use false horrors, but on on their own as you've tried, but as part of a mass fear strategy. summon Yazatas for their awe, get your casters to spam fear inducing spells, such as your False Horrors, Wailing Winds, Panic, Terror, Blood Rain, etc. as many of those as you can get your hands on pretty much. mix in a few thugs with Horror Helmets for good measure if you want. the key to this strategy is to use thugs or whatever to take the surrounding provinces simultaneously, so when the AI army retreats it won't have anywhere to run and will vanish.