I got mine in Interval 11 - Ground Zero
When you go into that big room and you have to jump down to find a health kit with the hordes of ghosts coming towards you, you kill them and the floor boards come loose. You jump down into the hole and go through the corridors. You meet 2 ghosts, kill them and continue forward. When you start progressing down the long corridor and you meet a horde of ghosts flying at you, just hit slow mo and mow them down with the MP50 you get. I was doing this on the extreme difficulty and was just trying to be cautious. I hope this helps you guys get the achievement! Good Luck!
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