Upset Rock is the sixth stage in Squishy Cave.
It may seem silly that the boss is Sir Seal. However, the Seal has a 10000% strength magnification! That way it has 250,000 HP and 15,000 DMG. It is even more deadly then J.K Bun Bun, Bore, or Shy Boy!
It is not a hard level, though. Mooth comes out in a pretty fast rate so you can get plenty of money in a short time. If you have trouble killing the Mooth, it is recommended to have some good anti-floating cats like Swimmer Cat or cats that deal tons of damage like Dancing Flasher Cat.