At 18th level you should have been hitting anything in Befallen for full damage. I'm 20th now, and NOTHING in there cons above blue to me. I've also noticed that spells get resisted depending on the mob. Dorn for example conned an easy blue and yet all my spells were doing one third damage or so. So, I believe magic resistance is innate in certain creatures and not in some others.

Zalbar
20th Wizard
Solusek Ro