KeenGamer: “Challenge Mode in World War Z this week is insane. Don’t go in without reading our guide first. Not only do you have the joy of dealing with a modifier that stops you from sprinting, but you have to get through the mission, slowly this time, whilst being relentlessly pursued by an invincible Bull. These zombies can destroy a team without the added effects of other modifiers, but with them it just becomes a hot mess.”
Source: N4G PC World War Z Challenge Mode Guide: Nemesis In World War Z