You can practice fighting Fatty Whale and the other Holo Bosses as standalone bosses by going to the right of the boss door. And for a few Fatty Whle tips, he only has a hurtbox when he's jumping, when he's spinning and going from one side of the arena to the other, you can safely attack him as much as you want while staying inside his sprite. Also, don't forget to use Bronto Burt for valuable chip damage when he's jumping over you, it may not seem like a lot of damage, but it does help.