About 5 players (and empty locations, if there are less than 5 players in range) are frozen in crystal prisons after the dragon raises its right claw towards the players. Players closer to the dragon are preferred to be encased and the range is up to the mortars in the camp to the south. The crystals are destroyed in one hit, which makes this achievement obtainable only during off-hours when fewer than 10 players are present. If too many are participating, only players right next to the dragon are frozen but also instantly freed due to the aoe directed at the dragon making it impossible to even see the frozen players. The 5 crystals that are spawned to heal the Shatterer do not count towards the achievement.
Rangers can get credit by freeing themselves with their pet.