For anyone having trouble, this is the easiest way to get this achievement.
This will take 5-10 minutes:
Using your main/existing agent, get the Clusterduck grenade, which DOES NOT auto-unlock:
The Vision (area)
Supply Point: The Vision High Rise
Jump down to the ground
Look around for green statue - it is right behind it
Then quit to the main menu.
Select that same agent.
Start a brand new world, and select "Rookie" difficulty.
Skip all cutscenes (hold X).
Disable the first 2 power stations. Enemies don't matter and you should be overpowered enough that they don't affect you.
Once you regain control of your agent after disabling the 2nd power station, the agency will be pissed and throw a lot of enemies at you.
Your minimap should have lots of red dots.
Jump down to the ground and you should be able to find 5+ enemies grouped closely together.
This is helped by the fact that the first area in the game is smaller (one main road).
All starting enemies also die with one hit on Rookie.
As soon as you see 5 or more together, throw the Clusterduck grenade and the achievement will unlock instantly.