To have a strong Mage sovereign I usually equip them with decent armor and weapons early on and utilize them like a warrior.  Level path of the Mage, grab any +spell damage or -mana cost traits.  Either level your chosen magic schools when you level up or use Steal Spirit to level them by killing off heroes that give you advancement in your schools.  

Also, focus on building your economy around mana generation.  As many Conclaves as you can support and grab & upgrade every shard you find.  By mid game you can be generating 20ish mana/turn which allows you to cast a few big tactical spells every battle or many small ones.   At that point your Mage sovereign starts to outpace the warrior equivalent.  

Mages definitely start out weaker (though not much if you use them like warriors early on) and by late game they're unstoppable.  Massive AOE spells for 6 to 10 mana when you're generating 30 or 40 mana is basically a sure win.  

A strong fire mage can deal tons of damage. If you are playing Empire also choose Death for the Soulburning spell. Focus your sovereign on spell damage and your empire on mana and spell research, and you'll have a strong mage.

Thanks for those suggestions.

So early on do your mage just like a warrior for better leveling. And quickly expand to get more shards.

Maybe my next game will have magic heavy map settings just to try out those mages. I will try:

- a Sovereign of Pariden (or custom) faction with Decalon trait.

- She will only be given life magic. All the other magic types will be learned with tomes later.

- The free points will let me choose lots of additional traits for early combat strength when leveling.

- I will research basic armor, weapons and mounts and buy those. After that magic and basic economy tech.

Ceresa is the most dangerous unit in the game, especially now that she can cast Dirge and Graveseal at any time.

Mages can be super-dangerous, but you generally want wargs, HP boosts (to keep them from being arrows/flamebolted), init boosting (same reason), and spell powering stuff.

Mages also make good support for warriors, esp given how strong defense is in general.

In my last game I got my sov to level 37.  Managed to get path of the mage and path of the defender, ie nuke plus crazy hit points.  He one shotted the Ice Lord and the Desert-turtle-looking-guy-who-grinds-mountains-to-dust, with a focus cast flame dart.  Late game Mages are amazing, and the elemental summons can be very nice too once you have a couple shards and can power level them further on wild lands or other rabble.  

A tip, I find that managing the rabble is super important to leveling your champs.  If you don't need to clear the camp, then don't, let it spawn guys for you to farm.  They get stronger over time and you get far more xp for mobs than for AI players.