Gentle Geyser is the second stage in Volkanos Volcano.
Battleground
Some Mooths would be spawned at the start, followed by the final unit - J.K. Bun Bun.
Strategy
Use at least 2 meatshields (Cat, Tank Cat, Samba Cat...) and have several anti-floating cats, like Kotatsu Cat, Shaman Cat and Swimmer Cat to defeat the Mooths and J.K. Bun Bun.
Also, you can use anti-red to quicken the process (Sushi Cat, Axe Cat...). Have a Bahamut Cat or Valkyrie Cat, or basically any other long-ranged cats in the game (Lizard Cat, Gross Cat, Nerd Cat...).
Walkthrough