As there are no strong peons to support the Berserkories, you can easily defeat them by stacking anti-Red cats with knockback or stop abilities. Just make sure to quickly kill each of them apart to avoid fighting several Bersekories at once - to assume this task, strong anti-Red units such as Crazed Fish Cat and Island Cat are recommended.
Alternatively, try bringing along the Gross!!! combo (Gross Cat, Crazed Gross Cat, Creepmallow), Apple Cat and Gentleman Bros. If you time it correctly, every enemy in the level will be permanently frozen by a stack of Gentleman Bros. and Apple Cats, allowing you to kill them at your leisure.
Another way to cheese the stage is to bring Dread Pirate Catley along. Just spam him (maybe with some Recharge Speed Up combos), and with good initial RNG, the Berserkories will be unable to get close enough to attack.
Yet another cheese method is to spam iCat, the True Form of Apple Cat. Beserkories will have no chance of attacking as they will be frozen indefinitely.
Another strategy is to keep spawning Cat Machine Mk3 and some support units to go with them. Cat machine is resistant AND blocks the shockwaves, therefore letting your support attack the berserkories without getting hurt.
- Sunset's Howl, Red Alert, Boiling Spring, all stages in Growing Red and Floor 11 are the only stages in the game to consist entirely of Red enemies.
- This stage is a reference to Berserkory's description where the sunset on his back awoke his dark powers.