Penetration and cast level. Also, blood vengeance.

[MaxWilson]

Don't forget Rune Smasher for an extra +2 penetration. And I think it's been mentioned that you can Teleport on top of large enemy armies, Master Enslave them, and cast Ritual of Returning (or whatever the version is that brings your whole army back with you). Alternately, you could just cast the spell that returns only the caster, and make sure you have commanders ready to move in and control your new army. Like, say, Disir or Eagle Kings.

-Max