What the achievement asks
Find a hundred treasures using a Scrying Stone in Nazjatar. Five points, account-wide, both factions.
A Scrying Stone grants a temporary buff that reveals buried treasures around you. A hundred of them sounds like a hundred stones. It is not.
Hoard the stones and go once
This is the whole trick, and it turns an achievement people describe as a slog into an afternoon.
The buff refreshes. Opening a Glimmering Chest while the scrying buff is up extends it, so a single stone can carry you through a long run of chests without ever expiring. People finish all hundred on two to five stones rather than a hundred.
The other half is flying. With flight you can cross between clusters before the buff lapses; on the ground you will lose it between them.
Do not spend your stones as they drop. Save them, and use the first one only once you are ready to spend an uninterrupted hour in the zone.
Where the chests are dense
Any part of Nazjatar has treasures, but three areas have enough of them close together to keep the buff alive without effort:
Start at the Drowned Market, work through the Coral Forest, then finish at Dragon's Teeth Basin. Keep an eye out for the Arcane Sight buff, which puts the chests directly on your minimap.
Every chest counts towards the hundred. That was not true when the achievement launched, and a hotfix fixed it, so older complaints about wasted openings no longer apply.
The rare that only appears here
Sandcastle is one of the thirty-five rares in I Thought You Said They'd Be Rare?, and it has no location: it appears by chance when you open a scrying treasure.
There is no better way to hunt it than the run described above, so do the two together. A hundred chests is a great many chances at the one rare nobody can go and find.