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
#13tony8669Posted 5/2/2013 1:14:31 PM
I've also found the weapon level has a huge effect as well (so it's not ALL about character level).
If you don't believe me, go fight a lvl 50 Terramorphous with a level 35 weapon but a 50 (or heck, even 61) character. It's not even going to scratch him, even if you're using a Bee.