Here is my 100% Achievement guide and full walkthrough so you don't miss a thing.
Another easy one and can be done anywhere.
When you get 2 enemies close together, start mashing the

button and pressing the

at the same time.
Thats the brain mashing psi ability and using your zap o matic at the same time to unlock this.
Go to 26:35 in my part 1 video to see where and how to unlock this
Hope this helps!
Part 1
Part 2