What the achievement asks
Stay under the effects of Claustrophobia for 30 consecutive seconds, while the Torment Claustrophobic is running. Ten points.
Claustrophobic punishes you for standing in a crowd. Five enemies close by and the debuff appears. Keep it for thirty seconds and you are done.
The method
Find five weak enemies and let them sit on you. Mawrats work. Skeletal remains work and often come in large groups, and the cauldrons that spawn them make it easier still. Stand there and wait.
At current levels nothing on that floor can hurt you, so this is genuinely a matter of standing still and counting.
The achievement fires the moment the thirty seconds are up. You do not have to finish the floor.
When they die too fast
The real failure is your own enemies dying before the timer runs out.
Take off anything that reflects or retaliates. Damage shields and thorns effects will chew through five mawrats in well under thirty seconds.
Watch for the Blessing Surging Power. Its rotating orbs kill weak enemies on their own, and with it up the achievement becomes close to impossible by standing there. Two answers: pick enemies sturdy enough to survive the orbs, or take yourself out of the fight.
Go invisible. Stealth, Camouflage, Feign Death, an invisibility potion, a Vengeance demon hunter's fear sigil. The debuff comes from proximity, not from combat, so it keeps ticking while nothing is attacking you and nothing is dying.
Immunity does not work. Crumbling Aegis and effects like it clear harmful effects, and Claustrophobia is one of them, so the timer resets. Verified by several people who tried it.
A wing worth trying
The Upper Reaches comes up repeatedly for this one. Its floors tend to hold large packs, and they sometimes hold two hostile groups busy fighting each other, which is the ideal case: a crowd that has no interest in you at all.
The rotation is the real obstacle
Blessings and Torments are fixed per wing and reroll when the daily quests do. You cannot pick the one you want: you check what is up, and if it is not there, you come back tomorrow.
They are not the same across the six wings on a given day, so check all of them before giving up. People find the torment they need sitting in a wing they never usually run.
Layers matter too. A run carries more torments the higher the layer, so if the one you want sits late in the list you have to go deep enough for it to be active at all. Roughly: layer 8 for one, layer 10 for two, layer 12 for three, and the last slots only appear at 15 and 16.
Where it fits
This is one of the four under Master of Torment, which is one fifth of Tower Ranger and the Shadowlands meta.