This trophy is for killing 50 enemies in one area.
I did this during Act 1: Middle East in the area just area you get the oil drum. Here, if you shoot the enemies and go into alert status you will get a stream of constantly spawning enemies from the doorway you entered the area and also some jumping over a wall on the left side a bit further up the street. Kill 50 of them and Snake will hear a flashback of Liquid saying "You enjoy all the killing, that's why.", he'll vomit and the trophy will unlock.
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