Ten k is an achievement/trophy requires the player to have played all missions (including Strike Force missions) with a minimum score of 10,000 points, which are gained over the course of the level through actions such as killing enemies, accessing special crates and weapons, and fulfilling certain objectives.
Normally 10,000 points is achievable the first time on a mission, though multiple playthroughs may be required to unlock every mission depending on the plot path a player may take, such as failing to rescue Karma resulting in the Strike Force mission Second Chance being unlocked.
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