Baseball Riot

As noted previously, to get an extra ball you need to knock out 3 enemies with a single shot. This means you'll need 12 enemies in one level for this achievement, and the only one suitable for that is level 10 in region 5. There are 4 sets of 3 enemies: one set in the upper left, one in the upper right, one set with an enemy in the middle and two in the lower left, and the last set in the lower right corner. The top two sets are very easy, but the bottom two can take some fine tuning. On the plus side, since you're getting extra balls along the way, you should have extra shots in case you mess up. Here is a video to show how it's done: