I just completed this quest after failing the Crowd Warming bit.
This is how I did it:
1. I unchained a character that could use the "Teleport" skill.
2. I moved the remainder of my party away from the area.
3. I put the solo character near the arch closest to the NPC who was currently in possession of the skull.
4. I used the teleport skill on the NPC and moved him behind the large tree that is located behind his stage.
5. I killed the NPC.
6. After killing that ONE NPC, I was able to access the head.