Kill the Catfish.

This achievement can be unlocked towards the end of the Volga level when you have to acquire a rail car from a factory at the eastern part of the map. While in this factory you will be navigating walkways above and around water that will occasionally reveal a giant Catfish but it cannot yet be killed. You must get to the end of the level where the rail car is and in a small room there will be the set up required to kill the Catfish.

As you look out into the water there will be on your right a bell that can be rung to attract the Catfish. Then when it appears, you need to cut the three ropes in the middle that each hold a body or 'bait' for the Catfish, dropping them into the water. When it is feasting, cut the final rope on the left had side to drop the twisted steel girders and debris into the water to deal the killing blow. You may need to approach the edge of the altar in front to trigger the death animation if you happen to be standing too far back.