Balloon Counter:
Luffy takes the hit, then explodes with the fireworks body. Its easy to PSA cause Marth comes with a Counter already.

I ssaw a video where he reflected a cannon ball doing this. And its a lot more fair then stunning people with a shout and you don't have to worry about the balance of it.
If you don't like that... then pinwheel would be nice here as well. From the video of him fighting Arlong (shark guy) he stuck his fee into the ground then twisted up, and spun around. Its stationary and would be a cool move overall. But I prefer the counter.
Also, on a side note: The actual move, Hanabi is will probably use the fireworks AND the gattling poly's... and I figured out a cool way to do it. I was frustrated earlier ccause I was at a

.. but that was because I hadn't slept. I've got my zzzz's in and I can think straight now.