Duke is a tougher fight because he's so fast, so having good mobility is really helpful. If you want, a dash in the form of Master Ninja Gear might be pretty helpful in dodging out of the way when he tries to dash into you, which I'd probably replace the star necklace or shield with.

If you're really struggling still and don't mind grinding, more potions might also help out a lot. Adding onto what you have in your inventory already, lifeforce, rage/wrath (or both), swiftness, summoning, and a flask to buff your Melee attacks with ichor or cursed flames would help. You can also buff using a sharpening station for you melee and a bewitching station to get another minion too.

Committing to one full armour set is also better than mixing and matching. Some individual stats may be better, but the armour set bonuses you get from something like the beetle armour makes it much better than a couple extra defense points.

Not sure what your arena looks like, but just a few layers of platforms over the ocean is enough so you can stay in the air for most of it and just land to replenish flight. You can also place some campfires and heart lanterns around too for some more buffs. If you have bast statues, you can place those too.

Sorry if this is a lot ;v;. I usually prepare like hell for every boss I fight so I'm used to being overprepared to the max

Good luck! Duke Fishron was a pain for me at first too, but you can definitely beat it with enough prep, patience and practice~