oh God, ive got 93 of these people in my establishment, nightmare. I feel for you.
When you have her in the office always have some one else with you, witness.
Document everything that is said and keep it in a ledger, mine are marked "cause for concern"
Establish an agreed work contract, what is expected of you as her manager to support her, what is expected of her as an employee, albeit the same as every other fecker else in the establishment, get her to sign it.
Try rotas to establish a fair and open system of delegating tasks, so that every one has to do them.
Find out the companies stance on mobile phones on the shop floor, print off posters, put them up in the staff rooms etc, they need to know that it is unacceptable to be texting... all of them, not just one individual.
You are not there to be nice, you are there to manage her, so get a hard head about it.
These diva princesses get such a lot of pampering it drives me fecking mad. They are there to do a job and do it properly, they cant expect to not be pulled up if they are falling short...not in this climate anyway.
I am a hard faced cow, but have had to be, forced into it over the years. I did start off nice. 