Thanks A Killion Achievement in Halo: The Master Chief Collection

As a cumulative achievement, this will most likely come naturally during your time playing the four campaigns and messing around in the multiplayer or in custom games. On average, each mission in every campaign usually houses anywhere from 150 to 200 enemies in total. Using this estimate, a single playthrough of all four campaigns will net a low total of 5,000 kills to a high total of 8,000 kills towards this achievement. 

You will be playing the four campaigns a bare minimum of around five times each for a multitude of different achievements. This means obtaining 20,000 enemy kills will be as simple as working on every other campaign-related achievement whether that’s in a solo playhrough or cooperative playthrough. On the other hand, multiplayer and custom game kills against online opponents also count towards this achievement, so splitting your time between the single Player and the multiplayer will help unlock this achievement with greater ease. Once you have amassed a total kill count of 20,000 enemies or players, this achievement will finally unlock.