What about imagining a situation and and how robin and franky(the ones favorite for the 4th strongest) do like a ship attacks the sunny go and only one of them is on?
When Robin is on, she'd make a shield, a giant hand or smaller hands and throw the cannonballs back after that shed make 1 huge hand and punch the ship, the ship sinks, over
When Franky is one the ship he'd launch rockets against the cannon balls, catch them and throw them back, gun them down and after that he'd turn into a boat and get there, kick the enemies butts, and than punch the ship which would crack and be destroyed, he'd launch more rockets, shoot a laser at the ship, or maybe launch himself (feet propulsion) at the ship and like a huge canonball destroy the ship
You should continue like how about carrying gold to the ship
After my imagination Robin is stronger