Well, mage is the most difficult class to play with.I'm trying to be pure for my Dullahan wifu! No thot shall get in my way!
I'm also doing hard mode by playing a mage. I don't know why playing a mage makes things so much more difficult (at least for me), but it really does. I think it's because of the lack of good armor at the moment. Even mage shield isn't that effective with a magic of 8 (or 7 I don't remember off hand).
Magic shield is identical to usual shield, as I recall, so it shouldn't guard you any better than usual one does. If the skill you use has 0% block, it doesn't benefit from shields at all.
Better armor is cool. On the other hand, heavy armor will slow down your stamina regeneration. You may be feeling afterwards as Mage doesn't have that big of Endurance.
Oil+True Fire was always the best attacking combination to end fights without getting that wrecked, but it was eventually nerfed by adding resistance to elements, so... if somebody finished the game like this, I would want to hear the impression it left.