For Chapter 1 - Start the battle by upgrading your Worker Cat. When the enemies come near the Cat Base, spawn a few meatshields and Axe Cats, they will be your stallers. Meanwhile, upgrade your Worker Cat to max level. When Baa Baas and Pigge appear, let them progress towards your base. When they are close enough, spawn a Lizard Cat. Support him with Gross Cats, Bird Cats and Fish Cats, along with meatshields.
For Chapters 2 and 3 - Send out a few Wall Cats and upgrade your Worker Cat to max level. When Baa Baas and Pigge appear, spawn your stronger units (Mythical Titan Cat , Lizard Cat, etc.) along with meatshields.