So there's like a hundred or so lasting Devils right? Like the weak ones, the strong ones has like 2 dozen of 'em.
And then an idea just occurred to me.
The Ultimate Magicians don't need the army at all, actually, they're a hindrance, they just have to enter Demon Territory up to a certain amount, like the stronghold they just found, and then spur up some Gate Magic and cheers mate, you got yourself 0 days tired of travel army into enemy territory in a question of a few hours.
The problem with this thinking is how big of Gate they can make...
But well, surely they could use Gate Magic to traverse a fucking river, eh?