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 (see the example below).

To complete a challenge that asks you to “defeat X Stunned monsters", you must kill the monster WHILE it is still Stunned. Do this X times to complete the challenge!

Note: You can Stun any monster of your choice. However, if the monster is no longer stunned when defeated, then it won’t count toward the challenge.

Learn more about: How do I Stun a monster?