Some spells, such as the Mini Mushroom (Snake Magic), give you a chance of Shrinking a monster. 

A monster is Shrunk when its size is reduced. A mushroom icon and a turn counter will appear in front of it. The counter indicates how many card turns remain before the Shrinking effect wears off (see the example below). 

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

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

Learn more about: How do I Shrink a monster?