Gears of War: Judgment

This achievement applies when playing as Locust in the Overrun game type.

The locusts:

Tier 1: Wretch - Ticker - Grenadier - Kantus
Tier 2: Rager - Serapede - Mauler - Corpser

Tier 1 locusts are available from the off and cost 0$, whereas Tier 2 Locusts cost money to spawn.

All kills can be gained in Private Overrun matches, they do not have to be in public games. Kills in private/casual/ranked all count.

View Progress:
Main Menu > Stats & Awards > Scroll Down to 'Overrun' > Hit LB & RB. Pages 24 through 31 are your stats for each monster, and they detail how many kills you have.

Main Menu > Stats & Awards > Hit RB for achievements tab. Find achievement at the bottom of the list (If locked)

Any kills achieved before the release of this downloadable content (23rd April '13) do count towards this achievement!

Edit #1
If you are already over 100 kills for each Locust killing one enemy COG will pop the achievement. Thanks OGxBillyTheKidx

Edit #2
If doing a private game of Overrun, if you start as Locust and play until you swap onto COG, you can then leave and the kills still count. Thanks Wolfie9985

Please leave a comment if anything is unclear

The ticker can actually get 2 kills in one go.. Can be in Private Match.
Have a friend or dummy controller to help. It's fastest with a friend.
The friend or dummy will be on the Cog team.

Go in a quiet place in the map.
And instead of exploding your "helper" bite him until he goes down and first then explode..

So the person who needs the kills, will be the ticker, and the other a random Cog.
I prefer to use Paduk, since he has the least HP.
Bite Paduk's so he goes down and explode.

Voila!
You get 2 kills instead of 1..

*** UPDATE ***

When working on getting the Wretch kills..

A quick way to do these, is combine it with boosting the "Don't you die on me" achievement.

Have two friends and yourself go in a Private Overrun on Terminal..
On the right side at all the first holes/generator, there's a bit quiet areas.
Down the medic one at a time, wait for Stim grenade to be thrown, Rinse and repeat

All of the Locust kills may be attained in Private solo matches against robots. Some are easier than others. If, like me, you like doing things cooperatively, then gather 6 players (4 with guest controllers) and let's get to work.

Have one player without a guest controller invite the others into a game lobby. Have them Select Versus > Private > Overrun > Map: Terminal with Mercy Rules set to 'YES'. The idea here is to run one player through their kills per match. Another player will work on Medic Revives at the same time.

At the start of the match, tickers will work to create a path to the COG. With the Medic off to the side, the Scouts will group up. Once the player doing the killing arrives, the Medic will continuously revive her teammates as they go down. I would suggest that the killing player work to get to the highest needed Locust as quickly as possible, but that is only a suggestion. Just remember not to kill the Pretty Red Head for she is the Medic.

While the killing is happening, the remaining Locust can finish clearing barriers. A designated Locust player needs to head to the e-hole and reduce its health to 6% with the Right Trigger. They then stay near the e-hole to finally destroy it once the timer gets under 10 seconds. There is no need to reduce the health of the Generator. A turn can take about 15 minutes to complete.

Once the match switches sides, the game lobby host may back out to the menu (taking everyone with them) to allow for the next match. If you don't need XP, there is no need to play as the other side. All revives and kills will still count. However, if you wish to keep the earned XP, you will need to play as the other side. Just have the guest controllers select Wretches and all jump the barriers on the way to destroying the e-holes and generator. Adds about 2:30 minutes to the match.

Progress is best checked by looking up the achievement: Main Menu > Stats & Awards > .

Please note that execution kills do not lend themselves well to Medic Revives. Some Locust do not offer alternatives such as the Corpser. Use the Grenadier's Lancer instead of the Gnasher. Sometimes it is better to melee (Scout and Kantus). Having revived players is much quicker than waiting for them to run back to your location.

If no one needs the achievement for revives, you may select any available map. Select the map Ward if a player needs

^{Above method still being perfected by BlackDragon3980, Peaches 518, Rainnin Death, KingNewf and NonKosherSalt. We will edit this solution or add in the comments any refinements.
Questions or comments are welcomed.}