First, there are 2 diffirent difficulties of advanced. 1 of them is pre-bigrock and the other one is bigrock+2cities. In bigrock advanced became significantly more difficult due to using more spamming methods. They arent essentialy harder for a skilled team. But on teams struggling with pre-bnigrock advanced, they simply get dominated as they can handle the huge projectile count. Nightmare is at the bigrock advanced level. The only real problem in ghost town is the spy+tank wave. Thats where you get a DPS test and when lacking players to attack the tank, you are not going to beat it. Without those tanks, or when they have less health, the mission is easier than advanced. Thats actualy the struggle you will face in bootcamp. Many of the more skilled players will not care about bootcamp since they want the loot from completing missions. The moment you even get just 3 of these, nightmare is significantly easier, and depending on the classes you use can even be a cakewalk. Well, nightmare became the community standard for single wave missions. Although its not the best mission setting, it can be good for some random fun games and allows you to perform things regular missions often dont give you (you rarely will see 10+ giants outside of nightmare modes). And this is also why you often need 6 players. The high health values combined with tanks are simply a DPS test which with less than 6 players can quickly become problematic. Against 22 giants at once you can just use 1 heavy to stall most of them them, with a tank you can not.