This is a community challenge and should unlock once the time finishes if the goal is met. Sometimes it can take a couple days after to unlock.
You need to shoot the enemy in the crotch region. Game will tell you if you nut shot them.
The nut shot needs to kill them, you can damage enemies any other way before you do it.
You can nut shot females.
Mascots are the easiest to land it on.
Good place to grind is during Prof. Genki's Minf Over Matter which has a bunch of mascots just walking around.