http://borderlands.wikia.com/wiki/Damage Its not noticeable compared to the original. I never looked at the tables but Skull enemies are killable, they just take a bit longer. In BL1 you didnt go after them because of the DR.
Scroll down to 'Borderlands 2'.
"Enemies three or more levels above the player take less damage." --- http://puu.sh/2EUyC