Alright, let's break this sucker down.
In a 1v1 fight, Arbiter will always win due to his sheer mobility. Cleansing Beam is made for Buildings, Infantry, and poorly micro-ed units. Arbiter can use his power against PoR, and PoR cannot return fire with his. Late game, the Prophet can fly away over a mountain and laugh as the Arbiter drops into oblivion glitch land.
The PoR can outrush the Arbiter with Elite Honor Guards. They will eventually overwhelm the Arbiter. So, super early and super late game, the Arbiter will get destroyed. It is in the mid-game, when you're Prophet only has Ancestral Perversion, that he get's rocked by rage. And Jackals are the only thing that can stop a raging Arbiter.
So, PoR can mass Jackals and Hunters and crush the Arbiter's army of anything. He can also rush him early and it's GG Arbiter.
Arbiter must use his powerful mid-game to his advantage. And use wraiths against the PoR's jackals.
Praise the Lord