Let me clear up this mystery!
There is actually a scaling mechanic that reduces your overall damage on monsters more than two levels higher than you, that is independent of accuracy and defense modifiers.
At three levels above you and higher, your damage against enemies is reduced by a flat percentage amount, regardless of glancing blows or defense. This can be tested by using bleed effects(which ignore accuracy and defense) against monsters 3+ levels above you.
Monsters even more than three levels above your character's combat level will reduce even more damage, scaling additionally for each level of difference until you are doing a very small fraction of damage to them even when you hit. This is why you're(presumably) having such a rough go of it on Adera Plains, which is a level 28+ area very close by the level 13-15 Forest of Adera area.
As to why this mechanic exists, it's most likely to prevent players from trying to grind on enemies significantly higher than them for EXP and loot.
Edited by huhbum, 15 April 2014 - 00:11.