[Szumo]

Takes just one turn, set mages on patrol so they're outside the castle and cast something that summons independent troops there (horrors, ghost riders, etc.).
You can also use Ankh to make sure Life after Death effect works.
And place mages near a poison cloud or chill creature, with orders to hold, so no need to cast any spell either.
Of course, you'll need to something that will kill the enemy, but not too fast.