What is the hardest EE or challenge you completed across all titles?

Mine would probably be the blood raven skin on ww2 nazi zombies. You have to playnom grosten haus. 20 waves with out taking any damage. Survive to wave 30 without going down. And stay til round 50 to kill the Brenner. If you really never played ww2 zombies don't speak. Wave 50 is close to wave 100 on treyarch zombies. That game was gnarly.