Okay in the fight while it doesnt make sense that mc didn't die - it makes sense how the Orc got one-shot: It is unSLASHable ie swords (the main weapon of an adventurer) will be useless. However, weapons such as axes, hammers, and greatswords which are used to deliver blunt force would work such as how mc brought an axe down on its skull with a shit load of force.
However, the axe should have broken on a skull idk and how it went clean through a shoulder blade idk either The only thing that makes sense in the fight is that orc couldn't take an axe to the head, which is fair. Only explanation - axe is from priest so priest blessed it or some shit so holy axe that gives def up and atk up
TLDR - HOLY AXE