What the achievement asks
Obtain a Box of Collapsed Reality by earning 7,500 Temporal Essence in a single Time Rift. Account-wide.
Where the rift is
Time Rifts open at the Temporal Conflux in Thaldraszus, where Soridormi stands at 51.5 57.1. They begin at the top of the hour and run about twenty minutes, so there is no point arriving at ten past.
This is a numbers problem, not a skill problem
Temporal Essence is earned by everyone in the rift together, not by you alone. Seventy-five hundred is far more than a thin rift produces, however hard one player works. On a quiet realm you can play a flawless twenty minutes and finish nowhere near the figure.
That single fact explains everything else about this achievement. It was routine while the patch was current and crowds filled every rift. It became the achievement people write forum posts about once the crowds moved on, and it is regularly the last one standing between a player and Taivan.
What to actually do about it
The fix is people, and there are three ways to find them.
Join a group before the hour. Players list custom raid groups in the group finder with names like "Collapsed Reality" or "Time Rift Ach". Search the custom tab a few minutes before the rift opens rather than after it starts.
Use one of the standing communities. Several cross-faction communities exist for nothing but this achievement and the rest of A World Awoken. Press J in game, search the community finder for the meta achievement by name, and you will find groups forming for specific hours.
Take the busiest realm you can reach. Being in a full rift on a populated realm at a peak hour does more than any amount of personal effort on an empty one.
Roughly five to ten active players is the figure people report as enough, provided everyone works. It is not a raid, it is a handful of people who all turn up at the same hour.
While you are in there
Do not stand still. Essence comes from tasks being completed, so an idle character in a full rift is a wasted slot for everybody, including you. Take whatever task Soridormi offers, finish it, take the next one.
The rift's tasks have a history of failing to register. If a task will not tick, abandon it and take another rather than fighting it, and if you reach 7,500 and no box appears, that is a known and reported failure rather than something you did wrong. Come back at the next hour with the same group.
Where it fits
One of ten under Closing Time, a piece of A World Awoken. The other nine finish themselves while you run rifts for this one, so bring them along: this is the achievement that decides how many hours the whole branch takes.