The easiest way to beat the Duke is to wait until after you have beaten the Moon Lord. Make a mining cart track that runs from shore out to about the point near the edge of the map where the background stops scrolling. It should be less than 200 blocks long. Any longer than that, and you risk the Duke despawning... which he does way to easily if you get off the screen he is on (even over the ocean). Bash the ends with your hammer so that you will bounce back and forth when riding in the mine cart. You don't need wings or platforms.
The most important potion you need is THORNS... the Duke will fly through you many times... so make sure everytime he hits you, he takes damage from it. Use the dragon (use a summoning potion and the bewitching table to get 3 segments) and the Lunar Portal staff (click it just over the middle of your track before you start). Your minecart will also shoot at the Duke so now you have your thorns, dragon, lunar portal and mining cart all fighting with you. I recommend using vampire knives as your weapon because you will receive constant healing throughout the battle.
Use beetle armor and try to max your defense (everything on warding)... use an ichor potion so your vampire knives drop the Dukes' armor as well Use the fire gauntlet, ankh shield, worm scarf, and star veil to increase your battle stats and reduce any damage you will receive.
That will be enough to beat him on normal... on expert you will need more healing... use a honey block inside a bubble wall somewhere on your track and you will constantly be under the honey buff... put 3 heart statues at each end of your track just above your head hooked to a one second timer... each time you get to the end of the track on either end you will get 3 hearts... use regeneration potions and ironskin potions... and put a sunflower, campfire and heart lamp on each end of the track as well... they also will help quite a bit.
... if you are still having trouble... place a NPC house just above the ocean near the center and have the Druid move in... she will provide an additional 8 points of defense for you and some other benefits as well...
If done correctly, you should be able to fish for the duke, start your cart, and then just concentrate on hitting the duke with your vampire knives. The duke will fly through you many times... and you will fly through those sharknadoes over and over... but you can take it. Have some life potions available just in case, but honestly, you should never drop below 450 health... or 550 if you use the lifeforce potion...
Good luck with it!
~Jim