I do agree the defense is really hard, though possible but extremely difficult. You are right it scales horribly cause of the afk people around the area. You can't have more than like 5 people around to complete it. maybe 2 at each shipment and they all have to be CC Heavyweights + Waystations. I got really lucky the other day and got it. It really shouldn't be that hard though.
I'm having the same problem in the city, all the achievements are bugged.