It's impossible to play on Hard

[mare911]

I am not an expert but i can say from my experience (currently finishing impossible mage game):

As mage you will have hard time until the middle of Verona (in terms of progression)
Once you get more powerful res (paladins) and access to high tier spells game will be walk in the park (most enemies will be shown as lethal/invincible anyway)

P.S. try to get no-lose and trap medals in early game, they are great benefit later