AssistG 20

Doing an assist requires you to be close to another team member, simply pull your Left Trigger and this will bring up button notations above your near by teammates which ever button shows up X, A, B, or Y and while still holding the Left Trigger push that corresponding button and you do a team assist unlocking this achievement. You'll probably do this a lot anyway but you can do this when you first take control of the team before you see any enemies.