Some spells, such as the Stun Club (Owl Magic), give you a chance to Stun a monster.
When a monster is Stunned, a ring of stars and a stun countdown appear above its head.
Defeating a Stunned monster means that you have to kill the monster WHILE it is still Stunned!
Note: If a monster has been stunned but is no longer stunned when killed, then it WON'T count for the challenge.
Learn more about: How do I Stun a monster?