Sweet XP (Insane) is the last stage in Weekend Stage/XP Stage.
When the enemy base is at 80% HP, the first boss wave will spawn: Squire Rel, Kang Roo, Gory Black and several Shadow Boxer Ks and Celeboodles. After the base reaches 40% HP, Bun Bun Black will appear as the second boss.
Lineup: Macho Cat/Mohawk Cat, Wall Cat/Eraser Cat, Paris Cat, Ring Girl Cat, Dragon Cat/King Dragon Cat, Jamiera Cat, Crazed Bahamut Cat, True Valkyrie Cat/Ururun Cat, an anti-Floating Cat and any decent long-ranged Uber Rare Cats. The best cat for the last slot is definitely Prof. Cat Jobs if you are positive that you can win, but if you're still not sure, use Crazed Macho, Crazed Wall, Lion or Crazed Giraffe to fill it up. Bombercat, Dark Lazer and Eastwood can help you a lot too, so feel free to modify the lineup if you feel like it.
Start by stalling Doge Dark, Gory Black and Those Guys with only Eraser Cat and Ring Girl Cat. Don't attack the enemy base. The first boss wave will spawn around 220,000 HP. If the Cats reach the base, stop spawning and let Gory Black and Doge Dark kill them. Otherwise, the first boss wave may overwhelm you. You should have a maximum of 6 Eraser Cats and 1 or 2 Ring Girl Cats on the battlefield by the time you’re ready to hit the base.
After your wallet is full and your Worker Cat is at level 8, spam all your Cats except for True Valkyrie Cat. The spamming should be able to let you defeat the first boss wave (spam of Shadow Boxer K and Celeboodle). You may choose to fire the Cat Cannon or spawn True Valkyrie Cat after your Cats begin attacking the enemies, or save them for the second boss. Money earned from defeating enemies should keep you constantly at max.
By the time you beat the first wave, your Cats should be attacking the enemy base again, and the second boss (Bun Bun Black) will spawn. He shouldn't be a problem if you keep spamming your Cats. After the Bun Bun Black dies, just attack the base and you are done. Depending on your Study level, you could earn over 520,000 XP in total after you win (340,000 XP x1.5 if you use Cat Jobs) and obtain the treasure (100,000 XP).
The entire strategy revolves around stacking cyborgs, so stall the first wave with all 4 meatshields and use your excess money on cyborg. You should ideally have at least 3 cyborgs by the time you hit the base, so that you can take the oncoming wave of enemies. Remember to continusly meatshield throughout the entire thing. Once the wave spawns, you should have a huge boost of money. Spam your cyborgs with this rush of money, and any other cats you may have brought along. If you don't have much money from spamming your other cats, prioritize cyborgs over anything else, since they are your main damge dealer.
Once the wave has been defeated, Black Bun Bun will spawn. Continue spamming cyborgs and meatshields (and anything else you have), and since Black Bun Bun will keep getting knocked back, your cyborg stack will (most likely) not die. Once he dies, you win!