After a long time I finally hit 100%. The multiplayer achiements in this game was really hard to get due to two reasons. 1) The player base is not that big. 2) The players playing have played for a long time, and are, thus, pretty good. I had given up on the multiplayer achievements, but then a free weekend helped a lot. Some newbs joined the game and I was able to get most. The last, host a 4-player co-op game, I managed to get with a friend and some random joiners.