I would recommend attempting the fight Post-Mech as it will be much easier and the weapons from Duke Fishron will still carry you through a portion of the game.

The most important part of your load out is the Shield Of Cthulhu, the expert mode drop from the Eye Of Cthulhu. This is a critical part in making sure you can dodge attacks and is a great counter for the 3rd phase.

I would also recommend the Blessed Apple (Unicorn Mount) to outrun attacks such as bubbles and the sharknadoes Duke Fishron summons.

For your weapon, the choice I recommend for the fight is the True Night's Edge, obtained from combining the Night's Edge with a few souls from each of the mechanical bosses.

For armour the frost armour is a great choice for the fight. It can be crafted from adamantite/titanium bars and frost cores, obtained from Ice Golems during a blizzard in the snow biome.

You should also get a warrior's emblem from the Wall of Flesh to boost DPS and an avenger's emblem, crafted from any Wall of Flesh emblem combined with a few souls from each of the mechanical bosses.

For potions you should have an ironskin and regenaration potion at least. Any other potions you have can help with the fight.

For the arena do one platform across the surface of the ocean and another high up above the arena.

Obtain frost wings by crafting a frost feather with 20 souls of flight. You should have the frost feather already if you got the frost armour but if you do not, farm more ice golems.

This part is simple. You need a truffle worm to summon Duke Fishron using a fishing rod.

Head to an underground mushroom biome with a battle potion and wait for one to spawn.

Once one spawns grab it with a bug net.

The Guide To Critter Companionship can be purchased from the Zoologist and while in your inventory will prevent you from game ending a truffle worm you needed to catch.

For phase 1 and 2 of the fight it is simple. Keep far away from the boss while using your true night's edge at range to do damage while using the shield of cthulhu and unicorn mount to avoid attacks.

You may need to practice the fight a few times to learn the pattern but once you have learned it, it is quite easy.

The best strategy here is to practice.

In this section of the fight Duke Fishron will turn invisible for a bit then dash into you. The pattern he follows is 1 dash, Then 2 dashes, then 3 dashes and repeat. In between the cycles of dashes he disappears for a second before beginning the next number of dashes.

To counter these dashes you simply dash into Duke Fishron with the Shield of Cthulhu as he dashes into you and the invincibility frames should stop you from taking damage.

Repeat this while using your weapon and you should win the fight!