A good opportunity to do this is in Mission 7: "Fly Trap". You can replay it through mission select at any time (under the pause screen where you view the map and upgrade your character etc). A couple of minutes into this mission, you will go to the Lair safehouse. You will encounter a large group of enemies. There are more than enough enemies in this area to do this. Note: The kills only count after the focus bar has been maxed out (build focus by running around or knocking out enemies). Your focus bar is allowed to decrease between knockouts. It just cannothit zero. Restart the checkpoint if it runs out. It is highly recommended you purchase all upgrades first and then replay this part through mission select. The only tricky enemies are the Shock Protectors. The quickest way to kill them is with Traversal damage. Attack them from a wall run or climb on an object and jump down on them. The other enemies are pretty easy to defeat. Every time you take damage your focus bar will decrease, and when you knock someone out it will increase.