The “Party Planner” achievement can be hard to get because it requires you be in an online party with a total of 10 or more players. I just got it luckily but here is a trick that can help you get it easier.
Halve The Requirements
When in an online party, enabling Co-op (local Co-op) counts as having an additional player as long as both players on that device are actively moving. I have been playing Co-op with a friend, but if you are alone you can move one character with a controller and the other with the keyboard at the same time. In my case the online party would have had only 9 players but because I used Co-op it counted as 10. A more extreme use of this feature, for the sake of getting the achievement, could be getting 5 players to each enable Co-op to get the achievement with half as many players. You and just one friend can count as 4 z 10 required players. I hope this helps anyone struggling with the achievement get it easier.
