First, what kind of footage it is?
In case its H264 or any "compression codec" you will have trouble keying, especialy with the kind of footage you have(with a lot of hair around her head)even with professionnal tool, those shots are difficult.
The first thing to do is re-export your footage in 4444, prores 4444 or Animation codec will do it (not magic, but could help).In your screencap I see it in the plugin window, "reconstruction sampling"try to put it to 4444.
You shouldnt key in Premiere, do it in after effect instead, with keylight.
Before buying this pluging I will test the other included such as Ultrakey, and you should try the Keyer from Phyx not the Fast keyer...Always do your key in 100% resolution. Watch a online tutorial about keying.
Consider reshooting your talent with a tight ponytail, no hair messing around.
But you really should watch a online tutorial about keying...