Search the map for a friendly Mangler that is roaming aimlessly. Getting close to it will cause it to follow you. Lead it towards Dragon Command in the corner to the right of the trophies onto a pad. It should get pulled up into the ceiling to complete the Challenge. Stepping too far away from the Mangler on the way will cause it to leave you, and it will occasionally shoot you with its arm cannon. Zombies are infinitely spawning and should be incredibly fast by this point, so this is a great time to use Undead Man Walking if you have it. Also, be careful not to accidentally kill the Mangler.