User blog comment:Damac1214/Thoughts on the Black Ops 2 Zombies series/@comment-90.213.52.106-20130710210952/@comment-25496444-20130712145333

Only Stand-in you can do yourself, and surprisingly you do get an achievement for it. Kassimir Mechanism, Time Travel Will Tell, Richtofen's Grand Scheme, Tower of Babble, High Maintenance and Pop Goes the Weasel (and probably whatever it is in Buried) all require multiple players. I hate it when developers favor people who can play with many friends.