In that particular game you would be crazy not to get as many different magic paths in order to utilize the huge magic site frequency.

I'd steer clear from Nature and Death since you have a perfectly acceptable mage of Nature and Death in the Pandemoniac. (Maybe take Nature up to 4, for getting Nature Gems ASAP, but death isn't a priority)

I'd look towards Air (since you can't make harpies), Astral (Magic Resistance and magic resistance gear is key) and then whatever else you prefer.

Use death summons like Banes for commanders (if you don't use pans) and other mages for summonable 'elite' units. Depending on what army you have your Mandragoras in (The Unholy 4 Commander) you will need other more survivable troops since most of your units, even the Behemoth looking ones, and the Ettin looking ones, have bad MR and lategame you will be staring large Groups of powerful mages in most armies.