Gears of War: Ultimate Edition 10

Load up Act 1 - Trial by Fire on casual difficulty and play until the first short cutscene after the stairs (1 minute). In the back of this area is an e-hole to be found. We use this e-hole ten times to unlock the achievement. Throw a grenade in the e-hole when it emerges and you will kill three locust. The game tells you in the right corner of your screen whether you've killed three enemies (clusterluck counter). After you've closed the e-hole with your grenade, reload the last checkpoint and repeat this process until the achievement unlocks.

When you kill three enemies at once using a grenade you will unlock progress to this achievement, shown in the bottom right from the screen. Just reload the checkpoint and kill them again until you get the achievement. There are some recommended places to do this, here is a video: