This stage starts off with three weak One Horns. However, once you kill one of the One Horns a 4000% Assassin Bear will come out as the boss. You might as well just give up right there, since it's set as the boss, so it knocks your cats back and destroys your base in one shot. The Assassin Bear has 22000HP and 120,000 Damage. Shockwave Cats can defeat it, such as Manic Jamiera, or even better, Cats in the Cradle True Form. If you somehow manage to kill the Assassin Bear, DO NOT waste any time killing the One Horns. If you take a long certain amount of time, another Assassin Bear will spawn.
Use low damage knock-backing cats, such as Dread Pirate Catley. As the enemy limit is 3, Catley, especially useful in a stack, can knock the 3 One Horns back behind the base without killing them, allowing you to destroy the base with no chance of an Assassin Bear destroying you.
The only cat you need is the knock-backer. You don't even need meatshields.
Sync One Horn together using meatshields. Use Awakened Bahamut once they are synced. Time with Maglev Cat and cat cannon. Use cat cannon and then use Maglev.
Manic Jamiera shockwaves with cat cannon will leave the bear at 1%. So you can either use the Cool Japan cat combo (Gacha) or upgrade Manic Jamiera to level 31 using Catseyes. (Non-Gacha) This relies on RNG, where you have a less than 1/6 chance of getting a shockwave. The Manic Jamiera needs to be at least Level 30 to kill the Assassin Bear for Gacha, while needing to be 31 with non-gacha.