Level: Tayari Plaza
At the very beginning of Tayari Plaza you are going to encounter a couple waves of Covenant. After beating them continue down the street and you will run into Chipps Dubbo and another marine. He will tell you to cut through the buildings to flank the Covenant. If the achievement does not pop after that, melee the Chipps (his buddy too if you want) in the head (knocking their helmets off) and they will say something like "hey, same side" or "it's the buddy system" and that is when I got the achievement to pop for me. It originally didn't pop so I waited and still nothing. After I hit Chipps and he complained, it popped.