Monsters vs Aliens

During the second part of Chapter 1, you'll be controlling The Missing Link. If you grab enemies with the B button, and then use the RT to fire that enemy at others, you get more level score and DNA points. However, there's one part you can boost this. Later on in The Missing Link's first level, where you're prompted to do two Leap Attacks, just keep killing the enemies on the circular platform until the following achievement pops, by using the press-X attack or the press-Y attack:
Once this pops, don't stop what you're doing. Keep performing the X and Y attacks, without using the Leap Attack. 50 enemies after the above achievement, the following achievement will pop: Again, don't stop what you're doing. Keep repeating the method and kill 200 more enemies. Once you've done this, this achievement will pop.