For Chapter 1 - Start the battle by upgrading the 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, start sending out meatshields, Axe Cats, Gross Cats and Bird Cats. If you want to speed things up, use Cow Cat and Fish Cat.
For Chapters 2 and 3 - Send out 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.