This solution is just to clarify the other somewhat messy solution.Anytime you see the word 'BAIT' with a number of points beside,that is a credit towards this achievement.Basically if the enemy is shooting at you or even focusing on you & your partner kills them,you get the credit towards this achievement.
NOTE:The word 'Bait' seems to be key for getting credit towards this achievement for some people,although not for others.Decoy DOES NOT COUNT!! Thank you SNYPER E & Figbender.Hopefully this will stop the negative votes.
It should come through natural play towards the end of your first playthrough.
Edit: Just a request to the negative voters,please comment as to why so I can be as helpful as I was trying to be.