Does the cat have somewhere she can go to where Pepper can't get to her? That's very important.
Are there any particular occasions when Pepper chases her? We found that Alfie would chase Tabitha when she went to go outside, and would even try to rugby-tackle her. But we found that if we kept a jar of treats nearby, and then made him sit (for the first few times holding his collar), told him "Leave" *and* gave him a treat, he would let her go and not chase.
Are you clicker training Pepper at all? It would be good to teach him "Leave" as well as "Sit" (the clicker can really help reinforce this).