To do this, you need to hit Up on the D-pad. Then press X to tag an enemy. You can press LT to zoom in if you need too before you hit X. Once a tagged enemy is killed by a team mate, you should get the assist.
You can check your progress in Skill Assessments > Power Modules > Spot Assists.
I 100% boosted this game, so how I went about doing this is, when I was on a team based game mode (not FFA), and it was not my turn to kill, but it was my team mates turn to kill, I would spot enemies. I did it while they were working on the Air Jump kills for the module achievement