[Alneyan]

Mistform+Mirror Image+Air Shield (if appropriate) is the standard defensive setup, and it works very well. Rainbow Virtues will add two more buffs, usually Ethereal and Luck, or the Earth protection spells. Then you can jump in melee, as the Virtue packs a lot of punch: that Flambeau is nasty. Alteration 2 is all you need to take most provinces, but risks are higher, and so I tend to concentrate on medium provinces until Alteration 3 kicks in.

The other way is to go in melee and cast Shockwaves (the AI will do that automatically when faced with swarms). This results in very quick routs, considering that Shockwave deals massive damage in a wide area, but I think you get less experience when playing as a spellslinger.

The melee setup also has the advantage of making it possible to smack down some enemy Pretenders, like the Vampire Queen or the Ghost King... but any of the three will be slaughtered when attacking anything flying and not casting any buff (well, the Vampire Queen will survive if she is in friendly Dominion). If those Pretenders are defending, it might still work out.

There is always a risk of getting into heavy trouble, mainly because of random spellcasters, but Immortality is not a guarantee of being 100% safe. If the Pretender finds itself in a province outside friendly Dominion (because the province was in friendly Dominion on the last turn, for example), that Pretender is at risk, Immortality or not. A Teleporting enemy commander is all it takes to kill your Immortal. It's not *that* likely to happen, but neither is getting Feebleminded by the independents.