[Saber Cherry]

Jotunheim can counter them. Rain, Jotun Spearmen, and imps do a good job... Devils and lava warriors have no shields, so xbows work well on them too. Atlantis and Tien Chi might find cold bolts effective, once the devils are at close range, and the mages are surrounded by a shell of infantry. Atlantis can also use poisoned armor to their advantage against devils, imps, and fiends that attack with claws. Ulm troops can probably fight devils OK, since their armor is hard to penetrate with no weapons... and any Death-2 nation should be able to overwhelm them with undead.

Staff of Storms prevents flight, casts lightning bolts, and in general is very good against devils... preventing them from slipping behind your lines and killing commanders and archers.

What nation were you having trouble with? I can't think of any that are utterly defenseless against 6 devils by turn 15, unless you were unlucky in the initial expansion, or took drain on a difficult-reasearch map, or something... I find mid-game devils powered by soul contracts to be much more worrisome.

I'm not saying devils are weak - quite the opposite; I think they (along with fiends of darkness) are way overpowered for their research level, magic level, and cost. But if you expect them, there are some ways to deal with them... though it's not usually national infantry, aside from Jotunheim and Atlantis. And Ryleh, of course.