Well there's different types, regular sized ones (melee, ranged, mage) and then the huge ones (ogres, Saurens?)
For some reason your damage just seems to drop to practically zero against them for your main attack and they can kill you in 2/3 hits. I can kite them for maybe 5/10 mins but in that time I've done perhaps 2% damage.
I have no idea how the level system works either you have white (similar level to you) then blue, orange?, red (progressively higher level above you) but then sometimes you have a +2 or +3 or +4 after their name, no idea how it works really.