When at the portal to reach Wart, pick up the crystal and advance to the portal. Carefully use the mushroom blocks and hit the portal with them. To beat Wart, get a vegetable, go behind him, and throw it while he is firing at you.

Use Peach for this battle. Get a fruit and avoid the bubbles that appear from his mouth. While he is harmless and motionless, hover over his head behind him. He cannot hurt you from this point. When he opens his mouth to fire bubbles, throw the fruit at him from the back of him. While he is stunned, fly back over to the front of him. Collect another fruit, avoid the bubbles, and repeat the process five more times to defeat him, most likely without being scratched.