Ok, I will be real here because why not. Don’t fight duke before Golem.
For melee, Beetle Armor with the Possessed hatchet or Influx waver is good. You should use an imbue potion duke is not immune to, and have the flesh knuckles (optional), Warrior Emblem, and mechanical Glove.
Rangers should use the snowman cannon with ~200 rocket III’s and shroomite Armor. (Helmet) A cheese strategy is to have a Minecart rail with the piranha gun. Potions include the Ammo Reservation potion and Archery potion. Accessories include the Ranger Emblem, Magic Quiver and sniper Scope.
For Magic, spectre armor (hood on mobile, Mask otherwise) with the magnet sphere and spectre staff works well. Potions include the Magic Power potion, Mana regeneration potion, and 1-5 Super Mana Potions. Accessories include the Celestial cuffs and Sorcerer Emblem.
Summoners can use the deadly sphere staff and the Staff of the Frost Hydra. Potions include the summoning potion. The summoner emblem comes in handy, along with other summoner-related accessories. Tiki/Spooky Armor is necessary.
For an Arena, use asphalt blocks along the ocean with campfires, honey and heart lanterns. A heart statue is a must-have if you are using heartreach potions.
Potions should include Ironskin, Well Fed, Regeneration, Endurance, Swiftness, Wrath/Rage, Lifeforce, and Thorns/Heartreach. Inferno Potions can pop his 1-HP bubbles he spews. Gills/Water walking potions can come in handy, depending on if you’re fighting underwater or without an asphalt bridge. 3-5 greater healing potions come in handy.
Accessories include a good pair of wings (preferably the hoverboard, for its fast horizontal movement.) If you are unable to obtain the Hoverboard, Frostspark Boots with Ice Blocks work fantastically. The celestial Shell, or any of its materials, is a must-have.
For the fight, run across the asphalt/water/ice while spamming your weapon of choice. Go over him with your hoverboard once you reach one side of the ocean. Once he uses his sharknado attacks, fly into the air and avoid those areas. You may need to go to the shore.
Once he reaches his second form, he will become faster and more mobile. This is essentially harder than the first stage, but the same tactics will work.
Duke Fishron is the only boss to have three stages,(the twins could count as having four) and he only does so in expert mode. Once he is below ~15% health, The screen gets tinted with a dark blue and his eyes glow yellow. He is very fast in this stage, and teleports, going near the player and diagonally lunging towards them, dealing massive contact damage. Homing weapons are a necessity here.
Once you’ve killed him, congrats! Go use your Duke Fishron weapons and kill the cultists, towers and then the moon lord! Yay!