[Mathulhu]

Invulnerable to damage on heavily armoured locations. Enough armour to get DR 10 is incredibly expensive and heavy but they are not immune to combat; heavy weapons still knock them down, grappling still takes them out of action.

According to LowTech Cheap Segmented Plate (the cheapest) across the whole body costs $3660.
According to LowTech Plate (the lightest normal) across the whole body weighs 110lb.