I've been playing on normal, so maybe I'll try a harder difficulty soon.  However, even with recent balance patches, it seems too easy to become neigh invincible by using either your sovereign or champion as a tank and outfitting them with all the best items you acquire. Then, with another champion playing a support/caster role, there's not much of anything that can stop you.

Here's a few of the issues I've seen that seem to contribute to the problem,

-There's a number of items that seem too powerful, or at least too powerful for the level at which they're acquired and usable.  Maybe a general review of items would be helpful. 

-Stacking multiple defensive items can make your champion almost untouchable.  Maybe there should be a cap on stats for your level.

-Put a cap on the number of "miscellaneous" items you can have equipped.  In my current game, I probably have half a dozen equipped on my sovereign.

-Maul is still overpowered.  On monsters like golems and dragons, it never seems to miss (maybe because they're very big and don't have much of a chance to dodge?)  I would suggest limiting the number of extra strikes you get, while also decreasing your attack with each extra hit.

-Sovereign/Champions still seem to level too fast at higher levels.  Though, maybe that's just because it becomes very easy to kill monsters.