You have two teams of wizards. In order to make a move you must change shape into a creature or thing that beats, defeats or otherwise trumps whatever the other side has just changed into. So you might get a sequence like

Wizards: Elephant
Witches: Mouse
Wizards: Cat
Witches: Dog
Wizards: B - A - T - H
Witches: Telephone

at which point I have to admit I'm stumped so presumably the witches win that round.