Using Rage as a Mage is not that important, IMO. I know Dorkus disagrees.
I agree that Time Back and Soul Drain are very important, but I don't think I picked Rage as a level up choice in the entire game and I still had 55 rage by the end, which was quite enough.
Laffwis, with soul drain it depends how much rage you have and how many other rage spells you like to use. I settled on 30% drain for 25 mana (I think) because 40% needed 50 mana, which was a little too much for me. But 40& would be nice.
And I disagree that Leadership isn't important too. I think it is very important to work on throughout the game, especially on Impossible, as the enemy stacks are already much bigger than your own. I worked on Leadership whenever I could and was able to raise it to 20,000 by the end.
Basically, Leadership (20,000), Intellect (33) and Mana (140) are your most important things as a Mage. Attack, Defense and Rage are less important. You will get all three of those from Targets/Training Dummy/Shrine of Rage anyway, Rage especially if you visit Demonis early. I would like to have raised my Attack higher that the measly 15 I had at the end, but I found a LOT of Training Dummies throughout the game, and had over 25 Defence.