Come on. It's not easy on impossible. I play Adventure mod on hard and that almost equals the standard games' impossible level but I think it's been okay so far. Can still do most fights with no losses but I take a few risks with my Marauders to get some extra gold.
The enemy heroes are the only real problem, with their constant attack spells that you can't predict and I play Warrior, not a Paladin. I used to play on easy but got bored with the lack of challenge. It's like walking in the park a sunny afternoon.
I dread fighting any of the later bosses. There is no way you can get away without big losses.