Use area-attacking cats like UFO Cat and Bahamut in this battle. You should be able to eliminate the Metal Hippoes and Duches quite easily with UFO Cat. Spawn Bahamut Cat once you have collected enough money to do so.
When you reach the base and your cats start attacking it, prepare for the 3 Shadow Boxers that will appear; they will tear through your UFOs. Protect Bahamut with stalling cats such as Wall and Macho so it can attack the Shadow Boxers. After defeating the Boxers, you now can attack the base with ease.