After hitting the Enemy Base, Sir Seal will appears as a main threat.
For Chapter 1 - Send out a few Tank Cats. When they die, keep replacing them. Meanwhile, upgrade your Cat Wallet to MAX level. When Gory appears, spawn your meatshields, Bird Cats, Gross Cats and Lizard Cats. When you kill him, keep focusing on sending out Fish Cats and Axe Cats alongside with your meatshields. Sir Seal should easily die.
For Chapters 2 and 3 - Use the same strategy that was used on Chapter 1, but this time, using the evolved forms of your cats would be a good idea.