To reach the top area where Phytops lurks, you need to use your whip to climb up. In places where a vine blocks the way, use the whip to pull out a spike and then hurl it at the eye in the vine. Be careful to watch for shadows, because sometimes Phytops drops blobs of poisonous liquid down at you. Don't let the blobs land on you.
Near the top is a propeller that you can activate with your whip. Be sure to keep the stylus on the screen so you don't let go of the whip.
Phytops will emerge, blocking your way with two large vines. Use your whip to grab a spike and throw it at Phytops's face. After you have broken the two purple blobs above its mouth, the monster's eye will open. Throw a spike at the eye, then when Phytops falls, use your sword to hit the eye repeatedly.
When Phytops gets back up again, run around to avoid Phytops's vine whips. Be careful, also, to avoid the poisonous blobs that Phytops continues to shoot out.
As Phytops becomes weaker, it will move more quickly and will shoot more blobs. Just repeat the process until Phytops is gone.