Valve has just recently made some additions to Mann Up mode in Team Fortress 2. While the original mode was already challenging, with a good team, it could be beaten on a consistent basis, with very few attempts.

Now there are two new modes, easy and expert. As I have beaten the regular (now know as advanced) mode several times, I haven't felt the need to play easy mode. I have however been playing expert mode. Now, not only is having the right team essential, as well at the right strategy, but their also has to be some flexibility. Failures should be met with changed in strategy, and while it may take several attempts, the levels can be beaten.

One other requirement is persistence. This has often been a problem as, some players quit after the first failure. But now, Valve has add some kind of punishment for those who quit early. I don't know what that punishment is yet, but I just try to avoid playing with people who bail suddenly.