I combined this with the Hornet's Nest achievement. In the mission The Loyalists (8) go straight to Havelocks room and grab the blueprint from the trash can in the corner. Then go to Piero, talk to him and give him the blueprint, and tell him to put everyone to sleep. Power up the pylon, which knocks out all the enemies.
After that you can simply drag 6 bodies together and using Bend Time, shoot them all in the head with a crossbow, or simply drop a grenade on them if you don't need the Hornet's Nest achievement.