Baseball Riot

To earn an extra ball, you have to knock out at least three enemies with a single shot. So for this achievement, you'll need at least six enemies in a single level. Note that you don't have to earn both extra balls on the same shot. You can do it with two different shots. The tenth level in region 1 is the only one with six enemies, so that's where you'll need to do this. Hit three enemies with the first shot and the other three with the second shot and the achievement will unlock. Below is a video for reference: