Shadow does not have impressive DR. However they have almost double the defences of a "Level 2" Ivory spiderling. The Level number seems to be generated almost entirely from the endurance of the creatures, which is silly. If you regularly miss something it's much harder to kill than a blob with five times the hitpoints but no ability to dodge. That's my main criticism of assigning creatures to a "level" if it is not even close to an indicator of how dangerous they are. XP seems to be awarded based on level, so xp is not commeasurate with difficulty either.
The bestiary makes no mention of the special powers of the creatures I have encountered so far. I would have expected the notable attack strategies of any beast to be one of the most important things to mention: Can teleport, will target casters over fighters, can entangle its foes from a distance, can poison its foes with a bite, paralyse an opponent with fear etc.