Just wanting to underline that cleveland's referring to Anathemant Salmanders, the priest/fire mages, not the red lizards
Definitively Yomi can do good against Abysia. The game allows some rock/paper/scissors considerations (thus much more complicated of course), and troops very proficient in melee and fire resistant are not what Abysia likes to fight, really.
Fortunately, you can recruit Holy2 Anathemant Salamanders and Holy3 Anathemant Dragons in every castle. You can build very cheap and fast-to-build castles in mountain provinces i.e. So if Yomi's sending you demons, having some though shielded troops to keep them busy while spamming Banishment, will let you do great harm. If he's sending non-fire resistant troops instead (brigands and... bakemonos too IIRC?), well I think they really can't manage to beat your tough troops even outnumbering them 5/1 if you help with strong Evocation magic. Research Evocation straight up to level 5, and every new level you reach you'll gain a new cool, strong spell. Then go for Conjuration 3 for Summon Phoenix Power to be scripted at the beginning for every Anathemant (except eventually the one casting divine blessing, for him, second place), so all the subsequent fire spells in combat will be stronger and cost less fatigue.
Also, placing a castle in a province with indie archers to recruit a bunch of them might help you if, like the gurus may rightly suppose, you're facing unshielded troops.
Then it's up to you to exploit further weaknesses of the enemy
