Sylvan is a very offensive castle, considering the initiative and speed of its units (you just can't run around much with Ents, their initiative is too low and their damage isn't great, but they have other advantages). In general, Blessings don't necessarily have to last until the end of the battle. A couple of key turns, and that's it; you can finish off the enemy without them. The Academy needs a spellpower hero more. And Phoenixes are tougher, and Shields are better, and there's a bunch of Elementals. Besides, they like to play around with Chaos magic, and without spellpower, there's nothing to do. As for battles, I'll say this: usually, everything is decided in the first 2-3 turns, and then it's just finishing off the remaining stacks.