Crackdown 2

Kaell has some good points but the distance wont work.

Once you've activated the first series of 3 points (Absorption Units) and you have worked your way down to the first beacon loctation you will find a UV shotgun right next to the spawn point for the beacon. If you have trouble finding it just look for the ammo points on your HUD itll be by one of those. When you fire the weapon youll see the UV round go all the way across the screen but it will only kill up close..itll fool you visually. Once youve called the chopper to drop the beacon the freaks will start pooring out. Just kite a few then turn around and start blasting. Fairly easy as long as you remember that the weapon is a shot gun so you have to be within a some what close range.

If you miss this on the first beacon point (say you do it on multiplayer and your teammate destroys everything), you can easily get this later.

All you need is the UV shotgun (obviously) and to then go after a "Freak Breach". About half way through the breach, let as many freaks accumulate as possible and try to get them into a funnel point chasing you. Then let em have it. When I did this I wasn't able to get all 20 at once, but if you get most of them with your first shot you just have to hurry and take a couple more out right as they spawn.

If that doesn't work, don't fret. Just drive around town at night until you get to a point where the freaks are overrunning everything. Stop before you get to the main cluster and charge in firing your UV shotgun. If it's a big cluster, you should easily get this.

I got this in the first beacon. Keep running around the beacon once it lands and you are defending it and only kill the freaks who are actually attacking the beacon (signified by a yellow mark above their head) and wait until a ton of the freaks are chasing you. Once they have massed up, get a lot of distance and blast them

The other solutions are using the UV Shotgun, but I got this with the UV grenade. It was in Green Bay for the beacon underneath the basketball courts. I jumped to the highest point where the attacking freaks are attacking from. I waited until the attacking freaks just started coming out and then threw a UV grenade near the beacon and got the achievement.

Probably the easiest place to do this will be Unity Heights or at a Beacon event.

Equip the UV shotgun, and just start shooting everything in sight!

As long as the place is packed with enough freaks, you'll have this easily.