Underground is the last stage in Stairway to Darkness, sub-chapter 21.
- This stage has all 3 Bun Buns sent out; White, Red, and Black. With their fast attack rate, they can break through the player's defense easily.
- Peon enemies with fast speed such as Gabriel (Angel), Ginger Snache (Red) and Squire Rel spawn. Shadow Boxer K also spawn as support. It has high strength magnification in this stage. (400% at 1-star difficulty) so beware.
- Long distance between castles (5200/3600), if the players spawn too much defense Cat Units, Not enough space to produce will occur.
- Note that if you sync the Bun Buns, the battle will become much easier. However, due to varied health and knockbacks, it is easy for them to get knocked back and will then proceed to plow your front line.
Above all, DO NOT CONFIDE IN Necromancer or any other single-target debuffs. They will likely hit the Hippoes instead and by the time any Bahamuts or ubers have taken care of the hippoes, the line would be pushed, and they would be dead.
Rather try units like Delinquent or Bodhistava who has area attacks (Can hit through the hippoes).
- Cat Line Up: Crazed Cat, Mohawk Cat, Eraser Cat, Crazed Wall Cat, Crazed Sexy Legs Cat/ Crazed UFO Cat, King Dragon Cat, Crazed Dragon Cat, Bahamut Cat and Ururun Wolf;
- Battle Items: Cat CPU .
- Begin the battle, save money and advance the front line, Bahamut Cat and Ururun Wolf must not be spawned. Adjust the production to have max money, and turn on the "Cat CPU" item when the Teacher Bun Buns appear (after damaging the Enemy Base)
- Cat Line Up : 4 Meatshileds, Paris Cat, 2 Long-Range Dealers, Bahamut Cat, Ururun Wolf, and an anti-Floating Cat.
- Stack up the Bahamuts and hit the base with the meatshileds wave. When the Bun Buns appear, start spamming the anti-floating cats, and turn on the Cat CPU, then start producing meatsheilds. Cat CPU will control the Bahamut Cat and your other dealers. Don't pick many meatshields at a time, because you will likely get caught on the "Not Enough Space" warning.
- Cat Line Up: Mohawk Cat/Crazed Cat, Eraser Cat, Jiangshi Cat/Commando Cat, Paris Cat, Crazed Sexy Legs Cat, King Dragon Cat, Crazed Dragon Cat, Holy Valkyrie Cat, Crazed Bahamut Cat, Ururun Cat.
- Start the battle with some stalling to build up the wallet. When there are a decent number of long-ranged Cats, advance to the Enemy Base and start the boss fight.
- Only spawn Valkyrie, Bahamut and Ururun after the Bun Bun Teachers appeared. The player will be overwhelmed by the "Not Enough Space" warning, the three latter Cats will be killed and the player soon losing ground, but the Bun Bun Teachers will die one by one with the continous wave of Paris and other ranged Cats.