Wizard – “Hope of Zephyr” [Campaign 6, Mission 5]

Zephyr - Banish! Banish!! Banish!!! Kills ALL creatures that were summoned before.

Absolutely true for versions 1.0-2.1.

However, in versions 3.x, it's better to take something more useful instead of banishing. It kills a negligible number of summoned creatures. Holy Word kills more.