Instructions
First Battle
Allow Mashtooth to defeat you as Monstar. You enter the battle automatically as Monstar, and it is impossible to defeat Mashtooth with this character.
Watch the cut scene and start the fight using Starfy.
Jump above Mashtooth as he charges at you and perform a "Downward Strike" when Mashtooth is under you. Do the Downward Strike by pressing "Y" and "Down" on the directional pad when in the air. Move quickly and dodge Mashtooth's lightning attack. He will occasionally try to pull you toward him with his breath. When he does this, run from him and dodge or destroy the rocks that are thrown at you. Rocks can be destroyed with a regular spin attack. Perform a spin attack by pressing the "Y" button. Keep attacking him until Mashtooth is defeated.
Second Battle
Avoid the lightning strikes while flying.
Attack Mashtooth by pressing the "Y" button. It takes two to three hits to reduce his health.
Attack until Mashtooth flies into the background. He will start sending fireballs at you. Do a spin attack by pressing "Y" when a fireball is close. This will deflect it back at Mashtooth and damage him. As his health gets lower, Mashtooth will start to do the lightning attack and fireball attack at the same time. Keep avoiding the lightning and deflecting the fireballs until he is defeated.
Third Battle
Watch the scene as Mashtooth shoots the Moon at Starfy.
Tap the "Y" button as quickly as you can until you push the moon off the screen.
Watch the scene as Mashtooth again shoots the moon at Starfy.
Press the "Y" button as quickly as you can. Keep taping "Y" until a cut scene triggers. The cut scene signifies the battle is over and Mashtooth is defeated.