[Shovah32]

1200 Undead shouldn't be impossible to counter at turn 25, assuming you didn't get a terrible start.

While going from your tests you may need 40-60 priests to kill those undead, they are not the only answer to the problem, although they do help.
Decent troops should be able to kill plenty of undead. Archers will do fairly heavy damage to unshielded troops and the shield guys fall quite easily in combat. Battle magic can do heavy damage, depending on your nation and of course rank-an-file soldiers can usually go toe-to-toe with undead for quite a while - maybe even indefinitely if you researched straight for nice buffs.