After you cross the channel in the sewers (Chapter 2) you will arrive in an area with a single shredder going horizontally in the upper part of the screen. In the beginning and throughout this area, Human-Piranha things will jump out of the water, follow you, attack you, and try to kill you. Have your Platform equipped.
Here you can make use of the shredder to basically shred them to pieces. You have to be in the shredder's way for this to work. Now, have a Piranha follow you, run towards the shredder, place a Platform in the water and stand on it. If done correctly, shredder will kill it. Do this with 5 Piranha's and the achievement will pop.