Use merely Anti Red cats. Summon Apple Cat to stop One Horn's quick attacks. Be prepared to upgrade your wallet since killing the One horns gives you lots of money.
Or if you are embarassingly stuck on this level, you can cheese it using Pirate Cat (Rare Cat) along with one meatshield, Island Cat, King Dragon Cat to kill the first One Horn that spawns, and since Squire Rel is the boss in this level, you could knockback the One Horns back to the base, making it cheesable. You could also use Jamiera Cat to kill the 1st One Horn, or to knockback the One Horns if you don’t have Captain Cat.
Spamming Elemental Duelist will win the level.