The Major League Player is for "Hitting 100 enemies with throwables"
All throwables count for this but I recommend using bricks as they can be picked back up and used a second time. Given how frequently you use throwables, 100 is a really low value and you will likely get this after 5 or 6 trials.
Also two handy things to note:
1. The in-game badge menu tracks your progress.
2. Leaving a trial after gaining progress still counts towards it.
Video guide for reference: