Try to get this achievement at the same time if you can:
The platoon system is slightly different the Xbox 360 version if you've played that before. To access platoons, press
while in the garage, then select "Platoon". From here you can invite people from your friends list to join your platoon.
That's the easy part..
Now, you need to destroy an enemy that is in another platoon in game. If you look at the roster in-game with the
button, you can tell which enemies if any are in platoons by a little number in a shield in front of their name.
You can either try to focus on those enemies, or just rely on chance to get you a platoon kill.
For myself, this achievement surprised me when it popped, I was just having fun with friends, but if you want to go for it specifically, that's how to figure out which enemies you expressly need to kill.