This is easiest done on either Initiation 1 or 2 co-op missions, as they do not have boss enemies and provide you with very low numbers of riot shield and sniper enemies. It will be a good idea to play with a full group of 5 players, so that your team is not overwhelmed at any point. The Initiation missions only last around 15 minutes so just hang back throughout but stay close to your teammates. The only way to get knocked down is when riot shield enemy charges at you. If you either get picked up after getting knocked down, or revived after getting kill, you will not unlock the achievement.