I did it yesterday.
But at first you should open the promotion info xml and change master_of_creation miscast chance to something like 8% or so because atm it is horribly awful (8% should be balanced)
Then you first have to go for divination. skill law and sun. this has two advantages:
sun 2 is a good defending spell,law2 host of the einherjar have guardsman and somewhat good strength. then with law 3 and sun 3 you receive valor (and aurealis) and master of law and sun (grants +1 holy damage and +1 xp per combat)
With master of law and sun, valor and general with leadership promotion you have +2 xp/combat,+150% xp combat.
Then you should go and find orthus axe if you dont already have it.
and undead are a bad choice because they are unaffected by valor
and mutate your caster so that he receives bonusstrength (conviction/strong/heroic strength/heroic defense) and NOT heavy.
good choices for omnipotent mage are: Amurites, Grigori, Kahd.
Good units: Gaelan, Kahd, Amurite Archmage (mithril weapons, hurray), Hemah
Thats what i think. Ah and i would recommend to sync the tower of divination with finishing arcane lore and have instant strength of will

. I had archmages in turn 190 or earlier on epic speed