I am sure you have tried to stop the fights... but something that sounds strange, but has worked for me...

Next time the agressive one attacks the victim, YOU go after the aggressor. Yowl and hiss. Corner him and clap your hands, grab his scruff and hold him flat while you growl. (Be firm and careful not to hurt, what you are doing is being the alpha of the colony and correcting the behavior as a t'ed off mother cat would) Let him go when it is clearly You deciding to. Go to the victim cat and groom her. For the next few days give them both love and grooming, but make sure the ex-victim is greeted first.

It doesn't always work the first time or if there is a big underlying issue, but it usually straightens out the problem.

Again, you have ruled out medical issues which should of course be checked first. Be careful and firm. Good luck.