Metal Gear Solid 4: Guns of the Patriots

When you kill more than 50 enemies in any act/area, a flashback will occur with Liquid from MGS1 telling Snake that he enjoys all the killing. This will result in Snake vomiting and losing a small part of his psyche. Do this trophy in a playthrough where you aim to get an emblem like Chicken, so there is no need to go out of your way for it. There are a lot of places where you can attempt this trophy, with the best places being the first two Acts. Specifically in Act 2: Solid Sun. When you reach the Vista Mansion area, where a huge bulldozer will go to crash the gate, you will find a lot of PMCs as well as Rebels. This is a good place to get the 50+ kills. If the PMCs aren’t enough, continue killing the rebels too. While the rebels won’t attack you if you let them be, they will shoot you, if you start attacking them. It doesn't matter what type of kills they are as long as you kill the enemies. That said even Stealth kills count for this trophy, although it will be hard to kill that many enemies unnoticed.

I have received some reports that people have killed more than 50 people in an act and didn't get the trophy. For me the flashback popped when I killed 50 people in an act and in the same area. So try killing all of them on one area.