I believe Valk did some testing with that and came to the conclusion that relative levels do not matter.
He also came to the apparent conclusion that either every mob at every level has the same defense... or that they all have 0 defense and just have more/less HP to make them "tankier" or "squishier".

I have done parsing on lvl 1 dummies, and lvl 50 dummies, same results.
It also seems to hold for mobs as well. Relative levels mean nothing beyond requiring accuracy, and everything has a base defense of 0.