Actually, the ideal situation (assuming that you aren't allowed to use magic traps) is one wizard leading a group of 20-30 fighter types with a few bards thrown in as well. For spells you want Invisibility Sphere, Stinking Cloud, and Fireball.

The wizard uses Invisibility Sphere to make his whole group invisible until they are in position, at which point they all ready actions to attack and the wizard casts Stinking Cloud followed by a Sudden Quickened, Sudden Maximized, Fireball. Most of the enemies are now dead and those that aren't are nauseated, which makes it easy for your soldiers to kill them.

Throw in a few wizards or use 3 Sorcerers, each knowing one of the 3 spells, and it becomes more repeatable.