Megashark has a few issues with Duke's defense. Don't get me wrong, it's very possible to kill him with the Megashark, but his health drops so much faster if you switch to the Tactical Shotgun. Use Crystal bullets; chlorophyte are trash.
How about magic items like the leaf blower? I have spectre armor I could use with it. I don't have any better magic items at the moment.
Tried again last night. 2 times. still using the megashark since I couldn't get the tactical shotgun. I was very close to beating him the first time and when falling from the sky I landed in one of his tornado things!! 2nd try was my son's turn and he did very good but died by the dang tornado things too.