If you stand behind a police officer and Deadites come near you, he will shoot it and you can collect the item that it leaves behind without wasting any ammunition. This also works near portals.

When you cast your strength spell, wait until the Deadites are close enough. If they are in the correct position, the little wave on the ground of your strength spell stuns the Deadites, and makes them easy pickings for the chainsaw or other weapon. This also helps save ammunition.