Mages start with the Winter Dance scroll. If you invested in magic spells you should easily have level 2 rune magic by that point, which would let you completely demolish his army. But you are a mage who isn't using magic spells, and you are playing impossible. The game should be hard

. Honestly you should be playing a lower difficulty level if you are going for builds like that.
Even if you don't use infinite crystal exploits you should have more than enough to get every important spell. Simply don't upgrade the spells you don't need. You don't need most spells. And you'll get Artifactor quick, which means every crappy item you run across is free crystals.