I'n going to try and get the KEEP IT CLEAN achievement, and since you need to kill 7 Moa (those are the ostrich-looking animals, right?) I was wondering what mission is the second mission: Noble Actual or Nightfall? You know the cutscene at the beginning of the game? Yeah, is that considered a mission since you can replay it in the mission select menu?
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Counting for achievements and challenges includes the intro level "Noble Actual" as number one. Winter Contingency is the second level, and also the level where you get the "KEEP IT CLEAN" achievement.
Nightfall is the fourth mission, and there are moa there, but you don't get the achievement for killing seven of them.