Get 10,000 kills across all game modes. This includes all Single Player and Online modes (even Private Player matches) at all difficulties. Basically, if you kill anything in the game it counts. The best place to rack up long kills is in the mission Riptide (2nd mission) in Act 3: Atlantis Rising. You can average 170-190 kills per match on the Casual setting. The best place to rack up quick kills is in the first misson of Act 1 Training, where you can average 70 kills in just over 2 minutes or so on Casual. To check your total kills, you can find them in three places: On the game's Leaderboard, after playing a round or from the "My Games" selection screen (formally Xbox Live Arcade) on your Games Library section of the dashboard. Remember that even if you lose the match, your kills count towards your total.