Again with stone skin you are not trying to hit the stony skin, your target are the squishy bits under it. Energy attacks go through DR that is how that shit works. Energy attacks can be resisted, the squishy evil outsider you used as an example would take 10 less from acid, but the door would take full full damage from acid. Not because he has DR 10/cold iron, but because he resists 10 points of acid.