To unlock the Fisherman trophy in Metro Exodus, you must progress through the Volga level until you reach the terminal section and crawl through a radioactive pipe. Towards the end of the pipe, you will see the catfish eating some mutants in front of you.
When you climb the stairs, you will see a large door on your left that can be opened. Before using the progressing through the level further at this point, head through the small door in front of you into the small room.
In the room, you will find a bell, three ropes that you can cut to drop bodies into the water, and a lever that when pulled will drop large chunks of metal. To kill the Catfish, simply ring the bell to get its attention, cut the ropes to drop the bodies in the water, and then when the Catfish is eating them, pull the lever to kill it with the large pieces of metal.
You can see Maka's video below if you'd like more detail.