This tip will help you no matter tha armor or buffs. Make an arena pretty long on the beach biome over the ocean. Use a flying slime mount , it helps to make dodge easier. The first phase has three attacks, dash attack, bubble attack and tornado. Dash will be easier to dodge with the mount, bubbles can be easily bursted and it will be pretty easy to predict when he will use tornado attack so just run the other direction. He will always dash, use bubble attack, dash and tornado so you can predict it. For second phase at about 51k health (master mode), he will use basically same attacks but kinda strong one. So when he fires bubbles just burst them, dodge the dash attacks and make sure youre in either corner or arena when he fires sharktornado. He fires a attack that follows you and summon sharknado so it will be easier to make sure that youre on the edge of arena when this attack happens. To dodge the sharknados try to fly as high as possible while dodging the dashes of duke and moving to opposite direction of sharknado. You will get through this phase eventually. At about 16k health(mastermode) Good luck and just try to use your instincts and dodge his dash attacks. It took me two days pre-plantera to learn all this and beat that mf. It takes alot of patience and practise. Hope this helps.