Anyway, question time. Is there a way to protect my army from banishment, or should I just create less demon-centric armies?
There's spells that can increase magic resistance, some of them battlefield wide, iirc (higher up in the research). Banishment damage can be negated (or halved? Something like that.) by a successful MR check, so if your demons have higher magic resistance...
Setting things up to kill the banishers might help, too, of course. Flyers set to attack rear, large AoE spells, stuff like that. Fairly sure Yomi's got access to at least death (and maybe blood?), so you might be able to sic some non-battle disease causing stuff on their armies, get the priest population headed toward death. Assassins might work too, if you've got access (and if you've got blood, or are willing to get
into blood, something nasty with that assassin heart thing, if it's still in D4, would be vicious.).
I'm playing as early age Ulm. I have a fully kitted out Tartarian cyclops that killed about 200 soldiers in one battle and I have other equally capable commanders but against the sheer numbers my armies always rout eventually.
Unless D4's changed it, one thing you can do is send stuff like that cyclops in
solo (or only supported by equally capable units). The rout is based, at least partially, on number of things killed (or something like the amount of HP lost by those things or whatev'.) on your side. If you've only got two or three guys that won't die, they won't rout. If they're in an army with a bunch of crap that
does die, they can and
will rout.
Alternately, if routing is the issue, get some morale boosters! Stick 'em in with the groups that are routing. If enemy chaff is the issue, get some of those mass AoE spells good at clearing them out. I'd personally just say screw the army and send in the SCs on their lonesome, though, especially if you're at the point of getting tartarians. Probably, anyway.