Valkyrie Challenge

Head to the spawn room and stand towards the left-hand side past the graves. A friendly Valkyrie drone with a green light should spawn in. Standing near it will cause it to move forward towards Dragon Command. You need to stay near it until it reaches the end point, avoiding death due to the infinite zombies. Undead Man Walking makes the step much easier as well. It is also very easy to accidentally destroy the drone, so be careful when firing. Once it reaches Dragon Command, the challenge is done.