I didn’t test this on a tactical mission where you cannot take more than 6 agents (though it should still be possible with grenades and/or juiced up agents). Spread out your agents on the map until each one has sight of one or more victims. Shadow them until each agent is in a position to get the kill. It is helpful to move your agents to a square next to where their intended victims will stop on the next turn. Enemy agents will obviously make this much harder, so try for all the non-agents first. When each agent is in position, use takedowns or suppressed shots while standing next to six enemies (100% to hit when next to an enemy). Make sure any agents killing enemy agents have a suppressed weapons that can kill the enemy agents, or kill them last – do not use a takedown on them as it will not kill them. Breaching can also be used to great effect here, depending on the map.