Your 8 people aren't my 8 people aren't his 8 people. It's easy to say no one should be able to beat it with less than 8 people, but realistically you'd be alienating most of the player population in the process. By default, most players are average in skill level. To raise the difficulty to the level you suggest would mean targeting only the most elite players and making it near impossible for them. Anything less would be mean they could more than likely accomplish the same with 7 people.