Man, I did it
I completly forgot on Ray of Light, which I was save-keeping for undead battles. Funny is, I was using same tactics as you describe from the beginning, just Ray of Light done its job
I destroyed all items for crystals and as for my skill build - I was interested only in mage skill tree, when all skills have been maxed out, I just upgrade random skills in other trees for which I had runes.
Thanks again, I though I will never beat this game. Tomorrow I will continue with other battles... yupi!