GLITCHED - While the trophy pops fine if you do it, in my experience playing in co-op was extremely broken and often unplayable, meaning I had to try multiple times before I could do the trophy properly.

To prank a co-op partner, you first need to complete a little challenge given to you by the game, which you can find by holding . There are a variety of challenges available from killing enemies in a certain ways to stunt related ones. If you get an unsavoury challenge (or if the game is too broken to complete it) you can quit the game and rejoin and you'll both get a new challenge. Once you've completed the challenge, hold again to prank your partner. You both need to prank the other person to both get the trophy.

If you need someone for this, see the Co-Op Partners Thread.