How to complete challenge x13?

The game Once Upon a Tower has a challenge that says in French "Faire tomber un coffret-cadeau sur un ennememi" or in English, "Drop a gift box on an enemy," but I don't understand what to do. How do I drop a gift box on an enemy?

The challenge is to drop a gift box on an enemy. Every day, you get 1 gift box at the start of the game. DO NOT DESTROY THE GIFT BOX! If you destroy it, you'll have to wait until the next day. You need to destroy the dirt below that gift box in such a way that it lands on top of an enemy. Note that you can restart the level as often as you like until you get a setup that allows you to complete the challenge, as long as you don't destroy the gift box.

A video that shows the challenge being completed: