Your mage tactics are adequate, I can assure you. It's simply very hard to deal with the enemies on impossible with poor spells and low leadership. I also have the bad feeling that the mage will keep on struggling even in the mid-game, until she pumps up the intellect stat a bit (by that I mean 30+) so that her spells can actually make a difference.
EDIT: Well, that was interesting
I spent the last 6 hours on a new mage hero, doing thieving sprees for maps and quests, and no battles whatsoever. I wasted over 4 days (now it's day 5, 8AM), but here's what I got:
All maps opened up but umkas, nameless and montero
Level 18
4000+ leadership (with only glory lvl 1, no items that boost leadership whatsoever)
120k+ gold (I made the mistake of purchasing troops - stupid idea)
15 T-Rex eggs
4 Black Dragon eggs
2 Red Dragon eggs
2 Emerald Green Dragon eggs
It is extremely irritating to get some of the maps, but in the end, it pays off. I restarted the game until there was a Verona map on Scarlet Wind next to the bridge. It is imperative for thieving sprees, as it opens up a ton of possibilities. Suffice it to say, the quests in Rehau alone (get a broth to improve Overseer Harus' pets - secrets of the temple - and Grass of Memory quest in the Temple of the Sleeping snake - you can evade the enemy hero that guards it) will net you a hefty bunch of levels. Other quests that can easily be done are the Dersu quests (make a gem necklace and get glasses for the old orc woman), the lizard's papers from rehau to verona and then to Elona (there is a shettera map there), then the quest for the demon's tongue (also nets a few levels), you will end up over lvl 15 for sure. Right now I'm 18, with a ton of runes to distribute, and I haven't even started fighting, nor have I done the quests on Rusty Anchor island.
Drawback: it takes quite a while to kite away the map guards. It also takes a ton of nerves to evade the enemy armies.
Positives: you start with good leadership (you had numerous flags for the taking), over 200k gold (even if you spend on a few items) and ready for everything.