LEGO Batman

You can get this on the "Face Off" level from the hero campaign in a couple of different places:

1) Just after you drive the truck across the bridge, you'll see a small group of four goons on the left of the screen. Keep going right just a little bit, and you'll trigger another small group of goons. Both goon groups will swarm you, and there are enough of them to get the achievement if you're fast enough. Make sure to wait until they're all right on top of you before you start attacking.

2) During the battle against Two Face in the bank vault at least one group of eight goons will attack at some point during the fight.

I got this acheivement on my first attempt of Hero 1-1. When you blow up the truck and jump over onto another screen. Immediately some thugs will run at you continue to run without taking them out and some more will run up the ramp and some off a little back alley (I can't remember if this is the first group that run at you) Smash them thugs away As quick as possibly and the acheivement should unlock :)

The earliest that i found you can get this while playing in Co-op on Level 1-1.
Right before you get the demolition suit build the rope to climb across to the Demolition suit. let your Co-op character follow but make sure not to fall down onto the street.
When you get across, you can look down below and you will see that there are 5 or 6 enemies waiting for you. and as you jump down there will be 4 more enemies come running after you from the door right under where you pick up the Demolition suit.
Make sure your Co-op character stays up in the lip of the building.
As soon as you get onto the streets all the enemies will try to attack you.Then all you have to do is to kill them as fast as they attack you.
There. You should have unlocked this very simple achievement.