XCOM 2
87 Achievements
Like Clockwork
Complete a successful ambush
79.7%
How to unlock the Like Clockwork achievement in XCOM 2 - Definitive Guide
To do this you need stay concealed. Organize your troops around a group of enemies so that they're at 65%+ accuracy. On the turn you're prepared to kill the enemies make 3 soldiers go into overwatch. Take your time with this as the first mission doesn't have a time limit.
Using your last soldier throw a grenade into the enemy group. The result should be at least 1 enemy being killed immediately and the others being left with 1 hp remaining.
With the initial attack completed the enemies will run for cover but will activate your overwatch. Once the enemies are dead the achievement will unlock.
Then I took a different soldier who was already in a direct line of sight and had her shoot dead one of the enemies. The second enemy took off running, and my overwatch soldier shot him dead. This earned me the achievement.
In fact, I got the notification for the achievement before the non-overwatch soldier's first kill's mini cutscene finished.
TL;DR I believe it only takes one soldier in Overwatch to kill an enemy who is scattering from a different soldier's kill.
Whenever you encounter a "pod" of enemies (usually a group of three), they will react by immediately scattering and taking cover. This can happen either on your turn, because you moved a soldier within the alien’s line of sight or the enemy's turn, if they are patrolling an area and spot one of your soldiers. In order to unlock this trophy, you must kill an entire pod as they scatter. To do this, your soldiers will need to have Overwatch activated. As soon as the aliens begin to run for cover (you will see a short cutscene first of them spotting your troops), Overwatch will activate for any soldier who has a line of sight on one of the fleeing aliens. There's a lot of luck to this and you will see many missed shots but this trophy will likely come naturally just because of how often you will be using Overwatch.
It is probably easiest to get this trophy early in the game when the enemies have the least amount of health. In fact, you could do this trophy in the very first mission if you willing to reload your save a few times.