Castiel
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I tried it, and after 3 attempts, I finally managed to win, although with huge losses (about 300 Wizards, several Titans, and Golems remained). The battle plan is roughly as follows: in the first round, use Priest, then unleash all the Mermaids, and finish off the rest of the weaker units. The main attack force consists of Nagas, which you use to attack with the support of Electric elementals. In the second round, use Mass Slow; the Nagas attack the Electric elementals, blocking the Mermaids. The Golems act as meat shields, absorbing the attacks of the Electric elementals and Fish. The Djinns block the Storm elementals and cast buffs on the Nagas. In subsequent rounds, simply throw Implosions at the largest stacks and heal your Wizards when the opponent casts Blind.
In principle, it was possible to win with fewer losses; I played poorly and rushed, which is why the result was like that.
My best result so far is half the Wizards and a third of the Titans remaining, but without Nagas, I can't imagine what to do in the next battle against the Legion of Magma elementals.
upd:
All the Wizards and Titans are left, I lost 50 Nagas and the rest of the meat. I think I'll stop here unless someone suggests a more winning strategy.
The idea was to drain his mana while he summons elementals, then use Mass Slow and slowly resurrect my units while the Earth elementals chase us around the field, but not kill them so that he doesn't realize he's in trouble. But for some reason, it didn't work out even once; either he casts Armageddon or something else irrelevant, and then he runs away :(
upd2:
I added screenshots. I finally did it :) It turns out that Berserk with Blind isn't the only thing that matters. In the very last moment, I stupidly activated it and lost 7 Nagas, but that's okay.