Now I am not sure if this is supposed to be like this, when you are in co op the more people you have in a world the less damage mobs take and they also do more damage. However, after extensive testing, I have discovered that this is reversed in the nether. The more people you have in the nether, the more damage mobs take. I have a farm of very high level (like 130 or so I think) wither skeleton spawners that I can change from item gathering mode to exp grinder mode by the amount of people in the dimension. If I have 5 people in the dimension at one time, they all die instantly. If I have 4, the tough survive but the rest die, and if I have 3, I use it as an exp grinder. I discovered this when I was building a witch farm for glowstone and was wondering why sometimes I would find a few high level ones alive in the catch bin but other times id be afk for hours and not a single one would survive. I am not sure if anyone else is aware of this, and if this even supposed to be the case, but as I have not found any other threads about it, I am guessing either no one else has noticed or they have because it is supposed to be like this.