I enjoy trying different strategies on the campaign missions to see if I can come up with a optimal way to approach them. We could have a pretty fun contest to see who can get the best time WHILE beating the 5 or 10 hardest missions. Since everyone takes different paths through the campaign we would need assume that all units are available to everyone.
Here are a few proposed ground rules.
1) No super weapons or nukes or Bertha on missions that did not include them
2) Pausing and slowing down the speed is allowed
3) Teaming up with shared units is allowed
4) Must be completed on brutal
5) Must complete all Bonus objectives
A few missions I have found difficult: