Crackdown 2

Fairly straightforward.

Find your first freak breach, (remember they'll only appear during 'nighttime') a yellow 'splat' symbol on your map... For me it was just across the highway and behind the first set of buildings opposite the docks where you get dropped off initially...

Approach the breach and hold 'y' to trigger the freaks spewing forth...

Just keep killin' 'em until (remember it's just the freaks with the 'splat' symbol above their heads - ordinary freaks won't count) you've killed enough that you get a message over the radio saying it's done... A helicopter will fly in and finish it off (the meter in the top right corner won't be fully depleted until then.) and the chievo should pop.

Throughout the game "Freak Breaches" will appear on the mini-map. They are represented with a yellow blob/splatter. Once you reach the area the ground will begin the swell and a hole will open in the ground. Freaks will start pouring out. Now the objective is to kill the Freaks with the yellow splatter icon above their heads. Kill enough to empty the meter in the upper right hand corner and the hole will be sealed shut by a chopper. The achievement should then pop up.