Play PvE Breakthrough, I prefer the map Manifest:
You are usually attacking in the PvE mode!
Equip your shield (left on the DPad) and go up to the objective on the ship and start to melee the enemies up there, you should get those 7 kills quite easy!
Watch out for enemies jumping down from the containers behind you, that's the only thing that might hinder you progress.
I got the achievement after 2 tries with this method!
Have fun!