You need to use an NPC is the major cities to Auto-Mentor down. You can unmentor at any point (when not in combat) but you need to return to the NPC in order to Auto-Mentor again.

So no unmentoring to skip trash. Still, for lower level dungeons, skipping trash is not necessary (Trash leashes, just run the adds out) (I do this right now with my appropriate level team in the low level dungeons, so I don't think its going to matter that much).