Do they really deal 400 damage at level six? I don't know much about Orcs then...

No, I don't think so at level six. I might be wrong, but something like that is possible around level ten. And I don't think magic can deal 400 damage at level six, unless it's a combined effect from an area-of-effect spell.
The point is that the Fury level, with a special perk, affects the hero's damage, increasing it by a factor of three. Yes, the Fury decreases with each round, and the damage is reduced... Plus, magic provides much greater tactical flexibility. But for a warrior hero, this is a perfectly acceptable way to speed up the early game.