My only critique is that you can't go back to get things if you missed them (I'm looking at you, chalices), especially spending 10+ hours playing the game and not knowing they were necessary to get the impossible achievement. Forced replayability is not the same as replaying for fun.
It's really frustrating when you get through three-quarters of the levels only to get caught by a single round with an elemental weakness that you didn't bring and a strength to the recommended element, and then getting smashed and losing.
Only sure-fire way I found to beat last-level Ninja champions was to max the two fireball talents (lower mana and more damage) and to max your mana to 500, then when the ninja comes just spam fireball on it 5 times; it's a guaranteed kill.
Self-targeting spells (like Juju's healing) shouldn't be able to be reflected; They don't even target the enemy. On that note, how can heals miss? Sounds stupid.