OS Graveyard is the fourth stage in IT Catacombs.
- Professor A. appears after 2 frames.
- Bun Bun Black appears after 26.7 seconds800f
- Teacher Bun Bun appears after 33.3 seconds1000f
- Because this stage only ever spawns 2 enemies at most, Teacher Bun Bun won't always appear at the time stamp, but he appears when either Professor A. or Bun Bun Black is defeated.
Money will be your main concern, so don't bother getting loaded with Ubers. Instead, use combos to help with money, and use cheap units.
Lineup: Mohawk, any form of Crazed Cat, Eraser, Roe Cat, Bombercat, Necro-dancer Cat, Hacker/Cyberpunk Cat.
Upgrade your Cat Wallet to 2, then spawn a Roe Cat. It can hold off Professor A. until Bun Bun Black spawns. By then, you will have enough cash for a Hacker/Cyberpunk Cat. Have Bun Bun Black kill the Roe Cat, then spawn Cyberpunk Cat, your 3 cheap meatshields and Bombercat.
As soon as Bun Bun Black is perma-frozen, stop spawning Bombercat. Hacker/Cyberpunk should be doing damage to the Professor A.
When money allows, spawn in some Roe Cats for defense against Professor A. In the situation when Prof A kills your Bombercats, just repeat the process. Eventually, Prof. A will die, and Teacher Bun Bun will come out. There, spam Necro-Dancers to permafreeze them.
Congrats, you should be able to either kill both and progress, or completely defeat the tower.
Strategy 2 (4 Star)
Lineup: Gross Cat, Crazed Gross Cat, Creepmallow, Hermit Cat (Mina or Souma are viable replacements), Sanzo, Li'l Eraser, 2 75¢ meatshields, Cameraman, Bombercat.
The first three units are present for their combo.
Cat Cannon: Breakerblast or Iron Wall
Try to stall Professor A. in the beginning with Li'l Eraser. When Bun Bun Black spawns, meatshield constantly. Wait for him to reach your meatshield line, then spawn a single Bombercat and Hermit/Mina/Souma. Then, wait for your Hermit/Mina/Souma(s) to snipe Prof A. Use Iron Wall/Breakerblast when Bun Bun Black gets knocked back, and replace Bombercat if he dies. If possible, stack Hermits/Minas/Soumas. Eventually, the Professor A. should die and Teacher Bun Bun should spawn. Now, start spawning Sanzo and Cameraman. Eventually, the 2 Bun Buns should die, allowing you to win.