advice with kitten attacking older cat

I adopted a kitten fairly recently, shes 5 months and I've had her since she was 8 wks. in the last 3 months (they were introduced properly and were fine until she started growing) shes been attacking my older cat (12, who I've had since a kitten as well) by pouncing and literally biting him in ass. she only does this with him, and it's because he WILL NOT FIGHT BACK!!! I dont understand it, he screams and yowls while running away but he won't bat or bite her back and i fear it's only encouraged her, kitty doesn't try this with my other senior because that cat corrected her. even more so confusing because I've seen him square up with dogs 5x his size with more effort he does this kitten. I've confirmed that no damage is actually being done, but I still have to break them up everytime they're in the same room because it's unnecessary stress for him.

she has quite a bit, as I have dogs that she is obssesed with and she doesn't really care for playing with us, shed rather the plenty of toys that she chases around the house making a racket 24/7. it's to the point where she no longer sleeps or goes in my room at all when hes in there and she cant find him anywhere because everytime I have to hiss (yes, actually hiss) at her to get her to quit biting my cat in the ass. im concerned its less play and more a show of dominance, but she does it everytime she sees him and it's getting ridiculous.

does anyone know how I can get this behaviour to stop? my only hard line is I won't be getting rid of any of my animals. I am open to the possibility of getting another kitten if you think it would help, is she possibly wanting specifically another cat to play with? my dogs are much larger so they dont really roughhouse with her, it's very one-sided on her end. she was very sweet to him before this, is it because shes now in a rebellious phase or what? the only times he fights her is if she tries to eat his food, and only if it's wet food, then she'll back off. they're separating through a transparent cat door majority of the time now, it's very inconvenient and cause stress for all involved (except kitty, who I think is very gleeful), any advice is welcome.