A very easy, but very missable achievement is for killing the catfish during the Volga level. You will eventually reach the train car terminal area, and crawl through a radioactive pipe, as the catfish eats mutants in front of you. Here, you can open the large door and then leave the area, but before doing so, go across the tracks in the side room to find three hanging bodies. Upon dropping them in the water, the catfish will come to eat them. Time it so that you pull the lever on the left side as the catfish is swimming up to eat the bodies in order to unlock the achievement.