After expanding Glitter Glacier for 5 pagies, head up to the front entrance to the Ice Palace. You will see Vendi in the first room, so I would suggest equipping the tonic that increases ammo from plants or extra health if you don't have it. Then head through the passage on the right and up to the passage on the left of the entrance. After a puzzle/platforming room or two, you will meet Burnie the boiler. After a quick chat, you can enter the boss chamber.
To defeat him, you must melt him. To start with, use the empowered sonar shot to break the blocks on either side of him. Then slurp from the flower at the back and start attacking him. At some point, it will be easier to hurt him with the torches in the back, so keep those lit and avoid getting hit until he is no more! He only has one attack, shooting snowballs at you, increasing in number as you damage him, but only attacks when still.