Extermination

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Kill 10,000 enemies in co-op

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This achievement will probably take anywhere from eight to ten dedicated hours to complete. There are so many enemies in the Co-Op missions, that on average, in a two hour playthrough, you should net around 1800-2000 kills.

The best missions to work on for this achievement are Missions 2 and 5. Mission 2 is very short, and since enemies are being spawned on both sides of the central area, you can split up the kills with your partner. On the other hand, Mission 5 has the most enemies which nearly never stop respawning unless you blow up all five fusion cores.

Whichever way you go about grinding this out, you can always check your progress in the Achievements tab in the Home Menu of your Xbox One.