[DrPraetorious]

Assuming you can get the death income, it's often a good idea to go for skull mentors as MA Ermor.

I'll expand on the advice that Meglobob and Aezeal have been giving. This is my most successful MP build for MA Ermor:
9S9W Imprisoned Oracle
3 Order
3 Sloth
2 Misfortune
1 Magic
Domstr 8

I think that balances out (I'm at work waiting for a colleague who IS LATE); if you've got points left over take a Growth.

An early expansion army should consist of an archtheurg and 16 shadow vestals. Put ten or so of the javelin legionaires *in front* of the vestals and set them to fire, which will prevent the occasional vestal casualty that you'd otherwise suffer from archery (legionaires are basically arrow-proof). That'll keep you going for a while.
EDIT: The shadow vestals should be set to attack. This set-up is resistant to archery but doesn't have much archery of its own to fall back on.

Against another player, put legionaires of some kind (we can argue endlessly about which kind is best, but the pros and cons generally balance out) in the front rank, shadow vestals behind them (set to attack rear) and undead in the rear behind your mages to soak enemies set to attack rear and to mop up in case the rest of your army flees for whatever reason.

Put the mages in the middle of the army and slightly off to the side - *never* leave them in the actual middle where they default because sometimes the computer mysteriously chooses to shoot at that spot. Spread them out in a line. Give them bodyguards that can't be targeted easily.

With Const 4, Evoc 5 and Thaum 2, you have battlemages that will punish enemy players well into the late mid game. Put ~three archtheurgs with skull staff (optional), eye, spell focus and *some kind of armor* and ~eight theurgs. The theurgs all form a communion and the archtheurgs cast shadow blast through it. This will often kill an entire enemy army, even of crazy stuff like angels. The armor, by the way, is an absolute must to protect your investment in these guys. Even a shroud of the battle saint makes them essentially immune to stray arrows, and much more resistant to assassins.