There is a challenge zone, where you need to parkour. Avoid being pulled by a smoker (you cant kill it, so you need to cut its tongue). In the end you need to walk on a plank while leveling chargers and finally it will give you a skull. I think that's all you need

when you're at the level chargers part i'd highly recommend trying to keep as much forward momentum as possible. the chargers respawn quickly. that part is brutal if you're too slow.