I used
yogurt for my chicken when it had a pasty butt and it helped to get out the bacteria. I noticed my hen pooping more, I trimmed the feathers and it was better after some time. You should check out the other post I made about this same thing and write your thoughts as well!
Do not use antibiotics, those take away good, and bad bacteria. In this case, the chicken needs all the good bacteria it can get, use probiotics. I don't know how to help with a skittish chicken, so I guess you will just have to work hard to catch it and put it in a place where it cant get out...
I recommend adding 1 Tablespoon/gallon of apple cider vinegar to the water daily for the next week. Apple Cider Vinegar is very effective in chickens and humans and is a great probiotic. Also, add a small spoon of yogurt daily for the next week as a treat. This is all going to flush out the bad...