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Complete 3 missions in a single day in Multiplayer Mode.

Online required. Can be done in solo private. As noted, the "kill x zombies" missions don't count unless they're given to you at the start of the day. Thus, this is only possible in episodes 2-4, as there aren't enough missions in episode 1. The missions you get are semi-random: there are a limited number of them, and if you play enough, you'll start seeing repeats.