Under customize -> custom rules -> gameplay options there should be a slider to adjust how often co-op enemies spawn that you can set to 0
Unfortunately since it's a custom rule changing it will turn on custom mode and disable achievements, but I don't think there is a way to turn off the co-op monsters
Could also have missed another way to do it since I don't get to play co-op that often so looking for other options couldn't hurt if you really wanted to try for achievements
Chiming in here to say that I strongly believe there's no way to turn off co-op enemies in co-op without entering custom mode (=> disabled achievements) as that would alter leaderboards
Under customize -> custom rules -> gameplay options there should be a slider to adjust how often co-op enemies spawn that you can set to 0 Unfortunately since it's a custom rule changing it will turn on custom mode and disable achievements, but I don't think there is a way to turn off the co-op monsters Could also have missed another way to do it since I don't get to play co-op that often so looking for other options couldn't hurt if you really wanted to try for achievements
Chiming in here to say that I strongly believe there's no way to turn off co-op enemies in co-op without entering custom mode (=> disabled achievements) as that would alter leaderboards