From the Super Mario Wiki
When the Fawful Express goes into the tunnel in the hill, the hill stands up and comes to life, revealing its feet, and sticks out its arms. It has two holes for eyes, and its large tunnel looks like a mouth.
If the player has Bowser breathe fire, the hill will catch fire and damage the Fawful Express on the inside. If not, the train will heal itself and restore a good chunk of its health. The Fawful Mountain will retaliate with a reckless charge at Bowser; the player must slide the stylus repeatedly in order to fend off the charge and send the hill into the spiked archway behind it for further damage. If the player doesn't counter the attack, the Fawful Mountain will knock Bowser into a spiked archway set up right behind him (despite it being destroyed a turn ago when trying to avoid the Fawful Express's missile attack). The hill's second attack occurs when it is on fire. It will summon several grey rainclouds with Fawful's glasses and teeth on them to put out its flames with rainwater. Bowser must shoot down the clouds with fireballs before they pour down enough water to extinguish the fire. He must also make sure to take out a few stray black clouds that float towards him before they can strike him with bolts of lightning from overhead.