Difficult Employee...What do I do?

I am having a tough time with an employee. Lately she has been giving me grief over the smallest things. She is on disablilty and is only able to work 60 hours a month. She doesn't want to work more than 6 hours at a time. She doesn't want to work 2 days in a row and the previous manager gave her a set schedule of Wed and Friday with one weekend a month. It has been tough, but I have been trying to accommodate all her requests. This past month while making my new department schedule I had a hole in the schedule I needed filled an no one else could do it. I asked her if she could and she replied, "Well, I usually take my art classes on Tuesdays and I like to spend my Mondays doing my art so I'll have to check but probably not." ARE YOU KIDDING ME???? I would love to sit at home and do other things too, but this is your job! Were it not for the set schedule I would have just scheduled her to work and been done with it! I am tired of catering to her wants and desires. I want to fire her, but I don't know what to do! Should I counsel her that this set schedule business doesn't work? Should I cut her hours down to nothing? What do I do? Oh, also, she has 2 worker's comp cases open against our company. They have been open for over 1.5 years and occurred under questionable circumstances. HELP!