I win.
I got bored by level 28, so I rushed through to the end. It's pretty funny that the 'boss fights' are very easy compared to the Overwhelming and Impossible stacks wandering around, but there it is.
I finished on day 36 with over 2 million gold. Score of
1274. However, there were HEAPS of things I didn't do this time, like close the Demon portal, completely clear out the Land of the Dead and the Gremlin Castle, and I virtually didn't touch anything in Murock at all. So I probably would have finished on level 30 at about day 40-42 if I could have been bothered.
Looking at the stats, it is no surprise that I killed stuff with spells (51%), troops (46%) and rage only 3% I barely used the ragebox at all towards the end, only Soul Drain. My best unit was Sprites with a massive 21.3% of total kills! When you consider I only reached Ellinia at level 19, you can see how overpowered a big stack of Sprites/Lake Fairies with Anga' Ring equipped is.
For the final fight I had red, green and black dragons, Sprites and Horsemen. I nuked them on turn 1 and finished on turn 2. Talk about anticlimax.
I think this thread has proved that not only is this game possible on Impossible as a Mage, it's not even that hard. I thought I would struggle with money but I had over 2 million gold at the end, and I wasted a lot on Armageddon when I couldn't be bothered to fight properly. So money just isn't an issue for a Mage, period.
I've really had enough of King's Bounty for now. I need a rest.