From the Super Mario Wiki
Like with Level 4-DK, the player needs to throw Polterguys at Donkey Kong to defeat him. Donkey Kong will constantly switch between which Polterguy is solid, while the other two Polterguys fly up and down on the screen. To reach Donkey Kong, the player needs to cause a Falling Spike to fall and act as a platform. Each time Donkey Kong is hit, the Polterguys will move slightly faster, and they will start to move toward the player after the second hit. Once Donkey Kong is attacked four times, he will be defeated and the player will move on to Mystic Forest Plus.