In impossible mode I don't see at all the mage as DGDobrev, I almost never use attack spell and feel them quite inefficient. Higher magic, more mana, int bonus increasing spell time length are great tools for some tactics a warrior can't apply as well. The point is to reach int 15 and 30 to increase time length of spells, and with more mana this offer much more choice than for a warrior.
For example:
- Start a fight with stoneskin and target at first round is a very strong tactic to use on units like Knights/Horsemen/Archmage/Cursed Ghost and against army with many long range.
- Use 2 traps at first rounds and more during next rounds can be very nice against army with many close range.
- Use Stoneskin on two units at first round, or stoneskin and Peacefulness on one unit, or stoneskin and Divine shield on one units are also very strong choices.
- Teleport plus stoneskin or mass haste and stoneskin can be quite good.
That said I agree with DGDobrev that the rage bonus during fight but also the leadership bonus and warrior skills are tools that seems to make the warrior easier to play. But I'm not sure if it's because I don't exploit well enough mage specific possibilities.