This achievement has gotten considerably harder to do due to the "recent" update which nerfed the Sandman. However, it should be noted that using the sandman is not required, ie, you only need to kill or assist kill someone who has gotten a speed penalty, ie stunned. I suggest for simplicity sake to use the sandman in a 2fort game or any other map which has more beginner players. Wait for the right time to hit an opponent with your baseball, such as when they seem to have taken a lot of damage or are about to take a lot of damage from your allies. It is important to note that this will take awhile.