Hello,
One of my players has Orcs and Giants as his favored enemies, and recently hit 8th level. So far, he's been using ferocity in combat to keep himself awake and fighting below 0 hp. He recently acquired an amulet of Natural Armor +1.
Does the Amulet stack with the natural armor +2 he can get from his Infiltrator Ranger archetype versus giants?
Since the amulet gives an enhancement bonus to your natural armor, and the Infiltrator archetype just gives natural armor, they will stack.
Yes. The amulet of natural armor is an enhancement bonus while the archetype actually provides a natural armor bonus.
My son is playing a ranger (infiltrator) with orc +2/giant +4 in GS, too, btw.
Edit: ninja'd!