You took the Aggro for 100 Flank kills
To successfully take the Aggro for a kill you need to have the enemies attention aimed at you. To do this, you need to keep shooting at them and use load powerful weapons, like shotguns and LMGs. While you have the enemies attention, your partner needs to then kill any that are shooting at you and Flank kill them. You will know when you have performed one of these kills successfully, as the word 'Bait' will appear on-screen informing you that you were the bait for your partners kill. Do this 100 times for the achievement.