Fish Out of Water - Achievement / Trophy Guide
In underground mushroom biome, search for truffle worm crawling on the ground and catch it with your bug net. Now go fishing to the sea and you will face fishron right after pulling the pole out. For the boss you can build arena of some sort. I did it after defeating the moon pillars, with Stardust dragon staff ( +spooky armor and all possible minion accessories) and solar eruption sword while going circles around him with my UFO mount.