Gates work quite effectively in battles if you level them up to 4. With a duel army, you can summon 9-10 Archdevils using the Gates (that's what I mean by scale). The level 2 Blood skill allows you to quickly accumulate enough resources for the first summon.
It's a shame that the Warrior doesn't have a way to protect the Destroyers; petrification would be useful on the first turn (tactically).
Overall, at first glance, things aren't so hopeless.