Hi guys:
You have to start the last chapter (6, THE PATH) in order to pick up the last ability, you can follow a guide but it's pretty straight forward. As soon as you get this continue the mission normally and you will see a juvie in front of you (shot him, don't use the ability). As soon as you pass that you will see a lot of juvies coming for you. Then activate your ability, go to the left corner where the juvies drops and use B button (knife). This will increase you barrier time and make your kills counts for the achievement. As soon as you lose your ability restart the checkpoint and repeat the same method. You should be able to make it pops in a few minutes.
Sorry for my English but is not my first language.