Has to be half-fiend for the reason of not needing food. This creature has been guarding the temple for a period of years without much in the way of intruders to eat. If it's too tough as is, could go with the Very Young White or maybe a Green Wyrmling. But I think if the cleric preps a Daylight spell to kill his chances to hide, and if the party figures out the temple layout such that they can either corner him or surround him, he's done.
He's clever but no genius -- Int 12 -- is not experienced at fighting "adventurers," and inhabits a relatively low-magic and low-level world. So he probably dealt with a straight-up incursion by a couple of low-level fighters and rogues without magic weapons (the temple's clergy was wiped out by his masters before his creation).
The party *can* bait him into launching a full attack by NOT hunting him and instead going straight for the Darkskull he's there to protect.