Gears of War: Judgment

This can be done in Private with three or more people.
Also works with one real person person and one extra controller on the locust team.

Cog Team: Cole
Locust team: Tickers (2 people or 1 person and extra controller)

Meet up on the left side of the Terminal map.

Have Cole fire Booskas at the two human played tickers.

Must be humans you are killing. Not bots. Rinse and repeat 50 times.
The more human controlled tickers on the Locus team the faster this works.

EDIT: For negative votes, please comment how I can improve my solution. It is the first solution with only two humans players instead three.

You can unlock it playing a private match with only one friend and a second controller.

It is faster because you only need two turns since you need to help only one person (if he/she needs it too);

- The player with only one controller must to host the match, because you can't do it with a split screen. Host creates a Over Run private match, with casual bots and Mercy rule turned off.

- Your main account must be alone on COG team.

- When the match starts, choose Cole (COG soldier). Your friend and the second controller guest choose Ticker (Locust). Using Train Cole, go to the left side of the map, where you can see a lot of luggage. There is a air duct there. Both Tickers go to the right, enter the air duct and stay there to avoid be killed by COG bots.

- Choose Booshka. Aim the first Ticker and shoot! You will receive The Double ribbon. Stay there kill some enemies if necessary or cover if you have the second controller until both Tickers arrive there again. Repeat it and the achievement will unlock after 50 ribbons.

GENERAL TIPS:

- If the Locust team opens the E-Hole, do the same thing during the second E-Hole. You can find another air duct climbing the stairs, to the right. As Cole you need to go to the left, where the Arcade games are. The air duct is at the end of this Arcade games area.

- If times run out, you need to start another match, because you will switch teams.

A special thanks to my friend ll Bellatrix ll who helped me out to unlock it

This is a video where gamers are boosting it. It is the same idea, but in a different area and using 3 players instead 2.

Video credits: Achievaholics

Most of the boosting sessions on TA do have more than six people in it

with 5 people playing as locust 4 tickers and 5th guy taking care of the e-hole

Use terminal map,meet up on the left site where the locust respawn

One guy of the COG (cole) doing the kills and can get 40-45 kills in the first round,destroy the e-hole when time is about to run out 10-15 sec. start round two up and get the last 5-10 kills. When chievo pop up restart Terminal and do the next player in line .

With this method and 6 people you can do the chievo for everyone in just over an hour.

Most be human controlled tickers you kill,when the ribbon that say "double kill" shows up wait until it disappears or you will get triple or quad kills and in some weird way they dont count as double kills.