At the start, Squire Rel appears as a boss.
After hitting the enemies' base, Le'boin appears as a boss.
When Kang Roo and Gory comes, things become serious. Hopefully, you already managed to kill B.B.Bunnies by the time they came. If you did, then spawn Mythical Titan Cat. Don't forget to protect it with meatshields.
Once you kill Kang Roo, money won't be such a problem anymore. Spend that money wisely, by spawning meatshields, Sexy Legs Cats, UFO Cats, Dragon Cats and Mythical Titan Cats. When you hit the enemies' base, the player will need to have at least 2 Mythical Titan Cats. Now you will need to deal with a big problem - 3 Le'boins. Luckily, Kang Roos will give you a lot of money. Just keep spamming meatshields, Sexy Legs Cats, UFO Cats, Dragon Cats, Mythical Titan Cats.