During the story mode when you come across a Gore Nest normally there are Power Ups near by. Find the Power Up then destroy the Gore Nest which will spawn all the enemies. Grab the power up and start killing the enemies but DO NOT KILL THEM ALL. If you kill them all you will get a checkpoint. Instead when there is one or two left, just hit the start button and restart from last checkpoint and all your kills will count towards the achievement and the Gore Nest and Power Up will be back. Just rinse and repeat until you get to 150 kills.