After the first stage of Dragonfly, if there's a party of 4 or more people, 1 person who was originally attacking before now cannot hit Dragonfly (they just run into Dragonfly). This has happened everytime I've rushed Dragonfly in a Dedicated Server (the vanilla ones or Klei default ones).
1. Join a Dedicated Server. 2.Build a rock wall or cage and initiate the fight. Make sure you have a party of 4 or more and see what happens.