Tactics for playing as the Sylvan faction.

Ah, but that's nonsense. Mages aren't fools; it's not like a single hero can take them down with one shot. The warriors of the Academy have decent defense. But if you charge a fireball at the elven archers, that's where the real fun begins, of course, if there aren't any unicorns nearby.
I remember playing a 3x3 duel game as mages against elves. The elves are great warriors, and their dragons almost wiped out my army. But the mage's trump card is Summon Phoenix, and then cast Celestial Shield on that phoenix. That's when things get really interesting. I summoned a phoenix and earth elementals, and that was it; the elves died one after another, and the phoenix didn't even flinch: it has 800+ hit points and takes minimal damage – from 0 to 20, depending on the strength of the attacking creature. The elves can't do that.