Boxing Clever: Hard is the first stage in Box Cats Awakens!.
This stage is made much easier by bringing some anti-black units. Spamming Flower Cat will help control the black units, but you will need to keep spamming as Heavy Assault will kill them when they get too close. Crazed Gross or Crazed Jamiera can easily handle the never-ending crocs with wave attacks. You can spawn a few meatshields and a Flower Cat early on in the stage to max your wallet, but don't send in too many cats or you'll summon the boss before you're ready.
Heavy Assault is outranged by Crazed Bahamut but will be able to hit almost everything closer, so spam plenty of meatshields to keep Bahamut safe from the black enemies while you chip away at the boss. Ururun also helps in managing the wave of enemies and keeping them pinned at the base.
If you have any LD Uber, you can keep the enemies pinned at the base and she can finish the level for you before you beat the boss.