When you’re on the Hardmode, try summoning Dreadnautilus, the rudest marine enemy in the game. It lives only in red waters under Blood Moon together with more commonly spawned enemies.
Its attacks have a strict system, awareness of which can help you in the tough fight. After a short charge, it will attack you directly and then leave you just to prepare for the next strike. The monster will attack you from a distance, quickly rushing at you between shots. Finally, three red seals will appear in the sky, and three Blood Squids will distract you from the main foe.