To unlock this achievement, you must kill 5 units with a single use of a tactical operation during a manual tactical combat.
Note this can be done on ANY unit; friendly, temporary, enemy etc.
There are many tactical operations that cause damage in 1 or 2 hex radius.
You can use multiple tactical operations, as long as the last one you use gets the 5 kills.
The easiest method I can think of to unlock was creating 5 weak temporary units and using a tactical operation on them a couple times to destroy them.
I unlocked while playing as the Assembly using 5 Reverse Engineers in tactical combat building Constructs and using the "Shredder Bomb" tactical operation on them:
Turn 1: Reverse Engineers created 5 constructs within 1 hex of each other.
Turn 2: Used a tactical operation on the Constructs (It did 12 damage, they have 20hp).
Turn 3: Used the same tactical operation on the Constructs, they died simultaneously and achievement popped.
This took me 5 minutes after loading up a save I had during the Assembly campaign before my final battle with the AI.
This method can be done with any similar unit or tactical operation setup.
Keep in mind you can save scum in this game to reload at any point.