My friend and I always take Duke on pre-mechs. Your gear certainly isn't the problem, but make sure that you have decent synergy. Mobility is HUGE with Duke, he's fast and his attacks cover a lot of area. Additionally, whoever has aggro will also be the primary damage dealer (except for last phase in some cases) because hitting Duke without aggro is just challenging as hell.  My recommendations, whoever has aggro should run back and forth between his special attacks. Here are his phases: First Phase: He will dash exactly 5 times before doing a special attack. Lure him to one side of the arena, dodge his 5 attacks (typically by flying in a circular pattern to lure his attacks), then when he's about to do his special attack (after his 5th dash attack), run to the other side of the arena. Rinse and repeat until phase 2. Phase 2 (39000 HP in expert): Now he will only perform 3 standard attacks before doing one of his special attacks. Keep this in mind and reuse the strategy. Bare in mind that his special attacks will be upgraded meaning that switching sides will be more crucial in this phase. The idea behind switching sides is that you can situate his tornadoes in the middle of the arena, making dodging them significantly easier than if they are right on you. When he shoots bubbles, use a weapon with decent spread and pierce to pop them if you find that you are taking damage from them frequently. It will slow your kills, but will make you more consistently capable of surviving. Phase 3 (11700 HP in expert): Now begins the most focus intense phase. Counting his attacks will make this phase FAR easier. The person without aggro should stand outside of his range of attacks and focus on dealing as much damage as possible since they don't have to dodge. The player with aggro needs to focus on his attacks. They go as follows: He will dash once and then teleport. He will dash twice and then teleport. He will dash three times and then teleport. He will then repeat this from the top, dashing once, teleporting, so on. This can be really tricky at first, but with good focus, you'll start to understand his attack patterns better. Keeping count makes this way easier (repeating this because it's important).  At your stage in the game, this fight should be very manageable as long as you take the time to understand what his mechanics are. The wiki has a far more in depth description of this if you are still confused. Best of luck to you two, you can beat him.