What the achievement asks
Open 100 small treasures around the post-storm Forbidden Reach. Ten points, account-wide.
What counts
Every treasure type from Treasures of the Forbidden Reach counts, except the Forbidden Hoard, which has its own achievement in Forbidden Spoils. Duplicates count: a hundred Bone Piles would finish it just as well as a spread across the eleven types.
That makes this a volume problem rather than a checklist, and it changes how you should play it.
Buy the scroll, then never farm one spot
A Reliquary Scroll of Perception from Cataloger Daela costs 2,000 Elemental Overflow and marks treasures on your minimap for an hour. It is the difference between the two rates people report: about ten treasures an hour grinding Obsidian Coffers in the War Creche, against thirty to forty an hour flying the island with the buff up.
The War Creche looks tempting because four or five Obsidian Coffer spawns sit close together, but it is also packed with hard-hitting elites, and the numbers say a circuit beats a camp regardless.
The circuit that works
Treasures respawn in batches, not one at a time, roughly every ten to thirty minutes. The method players report best results from is to park somewhere central, such as the middle of the Stormsunder Crater, let the batch come up, and then fly the whole island in one sweep collecting everything at once. Five to ten per pass is normal, and thirty in twenty minutes has been reported on a quiet morning.
Between sweeps there is nothing to do at the coordinate, which is exactly why this achievement pairs with everything else on the Reach: run rares, run Forbidden Spoils, do a Zskera Vault, and come back when the batch is due.
Why you keep finding nothing
You are flying too high. Treasures stop reaching the minimap above a certain height. Skim the ground.
Somebody else got there first. These are shared containers. Once looted the icon vanishes from everyone's minimap while the object itself remains for about thirty seconds, so a treasure that disappears as you arrive is often still lootable. You also cannot loot in combat.
You are not going into the caves. The Caldera of the Menders, Froststone Vault, the Stormsunder Crater vault, the Old Weyrn Grounds and the central Creche all have interiors with their own minimap, and treasures inside them are invisible until you are in. Several players report walking into a cave for an unrelated reason and having three treasure icons appear at once.
Too many people. War Mode on, an early morning, or a low-population realm all leave more of the batch for you. Players have used the group finder to get invited to a quiet realm specifically for this.
Do this one before the type checklist
Aim at the hundred first and let Treasures of the Forbidden Reach look after itself. By the time you have opened a hundred you will be missing one or two named types at most, and hunting those two afterwards takes minutes. Doing it the other way round means hunting Morqut Provisions for an hour and then still needing ninety more treasures.
Where it fits
One of eleven under You Know How to Reach Me, a piece of A World Awoken. A hundred treasures also produces a large pile of Zskera Vault Keys, which is most of Door Buster paid for in advance.