Check your skins, you might already have the "Ghost Levi" skin that is the reward for this challenge.
Starting at 0/15, I just had a game where I know that knocked out 5 times (only 3 different people) with the bat, and I was rewarded the skin. When I went to check my progress, it was only 3/15.
I think it might be 3 DIFFERENT people, so you can't spam one person for knock-out progression.