KeenGamer: “This week, World War Z presents a Challenge mode that isn’t quite as impossible as last week, but it’s more than possible if you use this guide. Instead, you’re faced with never dying, but you’ll lose 25% of your max health every time you’re downed. To top that off, you also have twice as many zombie swarms to deal with. Oh, did I mention that the zombies also now won’t stagger?”
Source: N4G PC World War Z Challenge Mode Guide: Undying VS Undead