MipperArmada
Skeletron
All you have to do is get the death sickle from the reapers during the solar eclipse after defeating at least one mecha boss then farm for hallowed bars ( bully the mech boss (es) ) so you can craft the hallowed armor set which is awesome or immunity and dodging but has a 30 second cooldown ( after hitting a enemy with no cooldown active you will dodge the next attack in a good amount of time I think like 30 seconds) and make sure to get the melee helmet. now, on the beach- not on the oceon, make a house with a bed with platforms for the roof which is very important so you can escape the house quickly so your nurse does not die, the nurse can also heal herself very quickly so keep that in mind ( the bed can replace a chair) then set your spawn point there and move the nurse into the house. You will also probably want the winged slime mount which is awesome for many reason because it falls quickly and you can bounce off of enemys dealing damage ( not that much damage) including bouncing on duke. It also falls fast and its awesome for dodging attacks. Make sure, and THIS IS IMPORTANT, have a bunch of recall potions, an ice mirror or a magic mirror so you can recall to he nurses house to get yourself healed and you need lots of money I recommend just farming bosses so when you have an extra boss summon item or anything just kill that boss if you can. ( I farm bosses Ive already killed like the eoc or boc). For using the death sickle just swing at the direction duke fishron is at so when he moves toward you he gets damaged, also swing at the fishrons he spawns especially in phase two when they are really hard to dodge if its even possible without the hallowed armor. One VERY important tip is to heal when you are low on health even if its just the second line of hearts unless you can heal becuase you need to heal when you are low on health ( with the nurse), which is higher health than you will expect in this boss fight. Also you need to have full health or else this will be very hard ( you can have like 2 hearts away though from full but it might be a little harder). When you have the first row of hearts down and a little more in the other row down, recall to the nurses house where you set your spawn point. I think ive covered everything and I hope you will get some good items and stuff and that this guide was a big help.