Excerpt from Sir_Toejam's mage advice5.
divine death - while most folks frown on the divine spells, this will take out any creature in the game, excepting the black ring bosses, even at level 1 (kill a 7500 hp dragon rider with one shot!), especially when combined with my absolute favorite and most utilitarian spell:
6.
Curse (survivor path) - both of the curse spells are cumulative (i.e., you can cast them multiple times on a single character, and their resists will continue to go down - just cast repeatededly on them without stopping for at least 5-7 castings), but only this curse spell will lower spirit resistance, so you can use freeze, leech, or divine death on those high hitpoint, very dangerous critters.
This combination is very good for dragonriders. Divine Death kills normal imps at one blow - for mage/shaman their resistances have to be lowered.
There is one spell, as Setharmon mentioned, you need to proceed to the enddungeon - but this one you can get by staying polite from the black Dragon in the Wastelands.
Kiya
Mage guides started by Kaan, the whole link, click here, pleaseLocusInn listed all spell levels, click here