Малганус
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I was just playing against 7 AI opponents on Champion difficulty as Sylvan. I assembled an army of water elementals and magic dragons and completely crushed everyone. My hero was also incredibly powerful, level 28, with strong nature and death magic, making me virtually unbeatable. These magic dragons are much more effective than phoenixes; they deal massive lightning damage. The map is huge, with about 5 Sylvan castles, and the AI doesn't recruit units through portals. I'm giving my vote to Sylvan solely because of the ability to summon Devils and the lightning attacks of the dragons.