I'm going to come right out and say it... Pandaren rare spawns are an absolute BITCH to kill as a priest, their heal can heal them almost back up to full health at higher levels and our only ability to stop the cast is a 45 second cooldown, longer than the cooldown on the heal. But today I had an epiphany as to how to kill them much easier.
Its simple really, Use Silence to stop the first heal then use Yaungol Wind Chime the next time she goes to heal to interrupt the cast which gives your silence a few seconds longer to come off cooldown so you can silence the 3rd heal(which she will cast a few seconds later as the chimes don't put it on CD)
It worked the 1st time I tried it but I assume there can still be a chance of silence not coming off CD in time, but its better than 6-10 minute fights in shadow just to out damage her heals.