why not arrange some performance reviews. Think of areas of focus, key responsibilities, planning and execution, business requirements, team development, customer focus, ethics and itegrity, interpersonal savvy, composure, decison making (these are what we focus on at my work). You need to come up with tasks in each category they should be achieving and grade them on 1 -4 1 being NI. Do it for them all, and get them to do it for themselves. You also need to take a step back and do some shift assesments. iT is good if you give all the staff something to fill out that will allow them to find 3 things someone does really well and 3 things that are "opportunities" for them to imrove on.
You may find its just jealousy and feeling threatened that is causing this deep down. Where she might be brilliant at her job she might be rubbish at interpersonal savvy and team building which are really important factirs too. Also if the rest of your staff are deliberately leaving her out of conversations and treating her badly this is bullying and needs to be dealt with too.