You will need a friend for this.
Create a server on any random map. Open console and type in the command tf_forced_holiday 2. Change the map to plr_hightower_event. Pick any class and have your friend join the game and do the same. Stand on the cart until the game ends, then go to Skull Island, wait for the cage to lower, and jump in. You should get it, but this achievement has proven to be quite finnicky, so you may still have to experiment.