Erff is standing just outside Vlox's Falls with four Artisans (rangers). They continue with you, but cannot be attacked. After talking to him, proceed to the location marked on your map (near the river). Erff doesn't like this spot, so continue with him to the next. This spot is next to a large structure with extremely steep walls, but Erff runs straight up to the top. Talk to him, and he approves. Now prepare to defend against Destroyers.