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This boss should remind you of the world 2 boss, Necky's Nuts, now he's back and purple(?). You defeat him in the excat same way, wait for him to appear and jump on his head. The main differance in this fight though, is that after a couple of hits, another Necky appears and attcks you in exactly the same way. Just keep jumping on the purple necky's head to win a Giant Banana