Once enemies castle is damaged, Teacher Bear appears as a boss.
Start by spamming Wall Cats to stall enemies, meanwhile upgrade Cat Wallet a couple of times and save up your money. When Sir Seal will come close to your base, send out Mythical Titan Cat and Whale Cat. After that, keep spamming meatshields, Brave Cats, Sexy Legs Cats. One you hit the enemies base, Kang Roos will give you a lot of money. Spend it on Mythical Titan Cats and Dragon Cats.