Good wizard first few battles would do with
Fatigue against living units and with
Spark against undead(kill skeletons).
Enemies without stamina wouldn't attack you and their defence would be halved.
Here I had only 1 starting magic arrow and fatigue in all other slots.
http://img10.imageshack.us/img10/9020/screen9dw.jpg So, you get impenetrable wall and would need something to do damage. For starters I would suggest crossbowman as 5th unit in your group to get grasp of buffing unit with
magic weapon, bless, fire weapon and astral energy. To prolong phase where you can deal ranged damage you would need
slow and
web. When you become more experienced with AI behaviour, you can take slingers instead. They can have same passive as crossbowman(at 20+ level, but they grow very fast), have more dakka and can move for 2 tiles. But start with them is harder, you need to give them medals on enemies without ranged defence like thieves and zombies. Both are very dangerous for fresh slinger.
Magic weapon, astral energy are useful for healers. Restoring their ammo and giving them second turn.
wind shield->phantom form defence for back row (mage and healers)
So, I would recommend this spell/building order, because crystals are limited:
1. library
2. healers
3. If your healer managed to get early meditation - astral energy/web
4. Otherwise prioritise magic weapon/magic armour
5. At this point hire second hero (these schools are pretty solid for any hero)
6. Start to get crossbowmen
example of tough battle. It is not a mage, but he have only 5 units and way less spells.
7. Get either sphere of wind for slow and mobility ritual or sanctuary for bless/curse.
8. Upgrade illusions. Phantom form and sleep are extremely good.
9. At this point you would get better benefit from T2 units and third hero.
10. Get fire weapon spell (For ghosts, knights and executors)
Basically, this would be enough to win colossal shard on expert difficulty.
As you can see, with this tactic spell duration>>spell power, so mage/commander dual would be better than archmage, not even counting that dual class would be able to field more units.