Honestly, I dont see why you can't kill the prot? As the priest you just need to dps, and dispell sacred shield. A prot pally has to cast, you have a rogue with stuns/gouge/kick/blind/garrote all which can interupt a heal. Force the bubble, atk from behind so you cant be blocked, and lock that pally down. If the priest is doing damage to the pally while the rogue is on him, he will bubble.
He has 2 cooldowns that will realy do anything vs your kick, one of them is bubble, you just dispell it right away, the second is aura mastery on a 2 min cooldown that you can stun lock, blind or MC him through its durration. Thats it, once those are gone you have two min to just have fun with the prot. Where he should never even get a healer off. Unless the dps is just owning your face I dont see why you can burst the pally down. And shadow fiend hits for like 700+ at least.
Thats the best you can do vs a prot, is lock it down. If you dont lock it down you cant win. So you realy only have the choice of killing the prot pally and you have to do it before your rogue uses all his cooldowns.