Gears of War 3

A good boost spot if you didn't get this from playing both Arcade and Campaign is Ch4: Under the Shadow. You start as the Locust and can restart as needed.

This can be done on any difficulty on Arcade or Campaign.

This is how I unlocked the Achievement;

Play the (standard) Campaign on Normal. Holding LT & RT 'fires' the Kryll at the CoG - using the RS you can aim them at enemies (almost like bullet-time).

When you gain control of RAAM, ignore his retro-charge and just use this.
You have to work through 3 scenes where you and your team call-up Seeders. Once you get through all 3 the Act ends, and you will earn the achievement.

Should have no difficulty in obtaining this cheevo through natural progression without having to boost or charge.

Just hold down the triggers and aim at the COG's and move to another once each is dead until the kryll bar is depleted. rinse and repeat on all COG's