The witch is harder as a mage. As a warrior, you can run up to her, and then smack the shit out of her. With a mage, you need to weed out her copies, and then you really only get 1 or 2 shots at her before she restarts her cycle. Another reason why the game gets so much easier towards the end with a Warrior is because NO SPELL, not Soul Ray, not Fire Storm, nor Gods Wrath will EVER do as much damage as a decked out warrior two-handing a MEAT CLEAVER.
As a mage in NG+, False King Allant was also very hard... as I said before, ONE mistake and you are dead; I had to team up with a warrior to beat him. The warrior just ran up to him and started laying it down, soaking all the damage, and occasionally running away to eat some herbs. Yes, I was in the back, blasting the king with my strongest spells, but it wasn't doing nearly as much damage as the warrior's Meat Cleaver.
For me, the scariest boss without a doubt is Old Hero. Not necessarily the boss itself, but because of the amount of work required to get to him. That second grim reaper, with the female specters that shoot lasers out of their eyes is a pain in the ass.