Would like to find the person who started this whole Pelinal being a robot thing and palm them in the face for coming up with such a terrible idea. Being on topic though I'm sure the armor is forged out of some super rare metal and just imbued with powers from the divines. Hell, it might of been steel, think I remember reading that the elves couldn't break through his armor which would make sense if the divines blessed it.
We only call him a robot because it's funny. It started when MK referred to Pelinal as "Gilgamesh meets the T-800." I any serious discussion, Pelinal is always referred to as a human Shezzarine.
Also, the elves did manage to pierce the armor, but only by using weapons wrapped in varliance, whatever that means.