I personally was struggling to do this, and I’ve figured out a far better method with the guaranteed amount of enemies.
RequirementsLevel 1 - New Kid On The Block - Section Stryker’s Island
Game Mode:
During story progression
or
Free play
Achievement GuideSo from the hub hit the START button scroll down to view map and select with

.
Now press

and select level 1 “New Kid On The Block”, and choose
Stryker’s Island and select Batman.
First section to try: Smash up the Terminal and broken bricks and hit the

to activate the next wave of police. While doing this do not kil or attack any them, gather them up by running around all sections to bring them into the middle area.
There should be approximately 3 goons and 6 police total 9 minimum.Now with Batman tap

on an enemy character now charge at the rest of them, guiding your grabbed enemy towards the others ensuring you hit them all.
If you Manage to hit 5, pop achievement unlocked!!
Second section to try: Once you are on the bridge do not kill the first 3 police officers, just use Batman to pull over the bridge.
As soon as the bridge is complete there will be 6 more police that spawn at the opposite end.
Gather them up counting at least 6 then, Grab one with

and once again charge at the other 5 making sure you hit them all.
You must ensure your quick enough, if you are successful pop achievement unlocked!!!
Congratulations!!!
Happy Hunting
*If you find that your coop AI is killing the enemies you could possibly join with a second controller, if this helps you slightly awkward view but still works.*