After hitting the enemy base, Hippoe and couple of Sir Seals appears as a main threat.
For all Chapters - Begin the battle by upgrading your Cat Wallet. When enemies comes near your base, Send out a few Tank Cats/Wall Cats. Meanwhile, upgrade your Cat Wallet to max level. When Baa Baa appears, start sending out Axe Cats/Brave Cats. They will take care of the Baa Baas and Jackie Pengs. Although, spawn Bird Cat/UFO Cat when Gory comes. This will get you a lot of money, so just send out all of your basic Cats. But after that, focus on spawing meatshields, Axe Cats/Brave Cats and Fish Cats/Whale Cats. This way, you will be prepared to face all of those Sir Seals.