Don't forget that you get terror as your starting spell...That's extremely effective for taking independent provinces fast. Just give the sauromancer(one or two tends to be enough against weaker indeps. This doesn't work well on knights) some cover with swamp guards or HI.
If your pretender is good at death magic he can join in too, right from the start
(note that this is harder the more independents there are. I've mostly used this on strength 5.)