I played with my friend on the hardest difficulty until we failed at the mission "The machine spirit"
we tried again with lower difficulty with no luck, every time we would lower the difficulty on failure we found ourselfs on the normal difficulty and enemies were still hard as ever. we noticed that it didint matter if we tried to chance difficulty on mission start since it would start every time as highest difficulty anyway. we tried reloggin, reselecting what so ever. only thing that we could do to continue tru the mission is to solo it.

Edit: I found solution, and it was to give my friend the lead, and make them select other difficulty and start mission.