Underground is the last stage in Stairway to Darkness, sub-chapter 21.
- This stage has 3 out of the 4 'normal' 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, Ginger Snache and Squire Rel spawn. Shadow Boxer K also spawn as support.
- 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. Area cats such as Ururun Cat can knockback the Bun Buns back together, so that they are synced to make the battle easier.
Above all, DO NOT CONFIDE IN Necro-Dancer 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.
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)
Lineup: 4 Meatshields, 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 meatshield wave. When the Bun Buns appear, start spamming the anti-floating cats, and turn on the Cat CPU, then start producing meatshields. 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.
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.
Power-ups: Cat CPU. (optional, only needed if you can't spam fast enough)
When the Ginger Snaches appear, lure them to your base, then start spamming meatshields and cameraman, with meatshields being top priority. A bit later, a Shadow Boxer K. will appear. Continue spamming meatshields and cameraman, and the shadow boxer should eventually die. This will give you a whole bunch of money, which you can use to spam sanzo, the meatshields, and cameraman, until you reach the base.
Once you trigger the boss wave, three Bun-Bun variants appear. Don't sweat it if they kill your sanzo stack or your cameraman stack though, as long you aren't incredibly unlucky, you should be able to push on. Keep spamming everything, as this phase gives you TONS of money. Once one of the bun buns dies, that is your signal that you have beaten this stage! The other two bun buns should be at low health, and therefore easy to kill.
You can replace the crazed meatshields with other cheap 75¢ meatshields if you dont have them, although crazed wall is quite difficult to replace, as the bun buns will push extremely fast. Jiangshi works quite well in place of crazed wall if you have it, as it's survive ability gives you two for the price of one!