A bully cat can be a problem, especially if s/he is bullying another of your cats. Cat behavior is difficult to change, but not impossible. Here are some suggestions that may slow down the bully.
Spay/neuter all the cats in your house. Hormones can increase aggression and eliminating them can help.
Have a separate litter box for each cat. If possible place them in separate areas. Giving each cat it’s own litter box can stop some fighting.
Give each cat its own food and water dish. Feed the bully in a separate area. Train each cat to use its own dishes.
Make sure the cat being bullied can escape to a safe place. Cats enjoy climbing and having a couple of cat trees offers a safe haven.
Keep a water bottle handy and spray the bully when he attacks another cat.
Comfort the cat who is being bullied so s/he will feel safe and secure.
Bach flower remedies, in particular their Rescue Remedy may calm the situation. You can place a couple of dropper fulls in a bottle of water and spray areas the cats frequent. A dropper full in the water dish can help all the cats. Or put a few drops in your hand and gently massage on the cat’s head and ears.
Sometimes one cat will take a dislike to another cat for no apparent reason. If all else fails, consult with your vet and/or a cat behaviorist.