Wild Sting Fling
The fight takes place in a single-screen area, apparently aboard the Gangplank Galleon, which is mostly flat barring two small ledges slightly elevated on the far left and right of the screen. While no Buddy Barrel is broken on-screen, both Donkey Kong and Diddy Kong are available to the player from the start of the fight, regardless of their previous status.
The flying, apparently air-breathing aquatic creature flies in from a top corner of the screen and swoops down in smooth arcs. To register a hit, the Kongs must simply jump on it, which causes it to bounce off-screen and reemerge from a top corner again. A barrel roll or cartwheel is ineffective.
After each hit, it will become faster horizontally - but not vertically, meaning it takes a wider & shallower arc every time, often turning off-screen.
Once seven hits have been landed, Wild Sting is defeated, three bunches of bananas fall from the sky and the way to Kremlantis is clear.