This achievement can also be earned during free play in Manhattan. An easy place (and one of the only few) to find enemies is Rikers Prison Island, aka "The Raft". It is located towards the north-easternmost part of the map, out in the water. Head to the back side, between the water and the prison wall, and waves of infinite prisoners will come at you, allowing you to earn the achievement without the stress of having to play through missions again unnecessarily.
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