I finally did this. It took me about a week for my sim to get to 5 stars. It probably would have taken less time if I hadn't wanted her to hold onto her employee for story reasons. It was one of the most grindy things I've had to do in the Sims and had it not been required by the rules of the challenge I was doing, I would not have bothered.
My sim had level 10 vet skill. I don't know that the level 10 charisma she had helped. Mostly I did as @CyrusTheLoser advised. I had the rate at 85%, the first level of advertising, higher quality supplies, and about a total of 4 patients per day.
