Take, for example, "High Tide" (Aeon mission... 4 I think?)
These guys did the mission in 31 minutes. When I did it with a buddy, the last objective (Kill the enemy ACU) started at 30 minutes in. Then you have to kill that ACU and three bases, and THEN kill that ACU again, all with huge numbers of enemies guarding each base. How do you do it so fast?