Have you turned on "Last Enemy" in difficluty settings tab? Do that and then enter the tactical map, speed up time to 7.5x by pressing the "+" button and wait until a "?" appears. Thats the enemy, go there and kill em.
If you don't mind having the name "Cheater" floating above your merc for the rest of the game, open the consol and enter: killenemies
The speed up time function is very useful in the game, it saves a lot of time running across a map or when you sneak up on enemies. It's the most used function in my game, after shooting oc
