What the achievement asks
Open ten Rift Hidden Caches during the Night Fae assault on the Crucible of the Damned. Five points.
The caches only exist inside the Rift, which is why people conclude they are not there.
You do not need the quest
The obvious route is the Night Fae assault quest "A Shady Place", which phases you into the Rift. That quest is drawn from a random pool and may not appear for weeks.
You can ignore it entirely. Take yourself into the Rift from Korthia and walk into the Maw while phased, and the caches are there regardless of what quests are up. This also works after you have already finished the assault's quests for the week.
The route in
Use a Repaired Riftkey at a Korthia rift portal, or a Collapsing Riftstone to open one on the spot.
Cross into the Maw on foot while the Rift is on you. Once the Night Fae Assault banner appears, use the Cypher of Relocation to jump to Ve'nari's Refuge without dropping out of the Rift, then ride south-west.
You have about fifteen minutes inside the Rift, and the Cypher has a twenty-minute cooldown, so plan the trip before you take the key rather than after.
The caches
Expect two days
Six is the number people find in a single day, not ten. The caches have a daily lockout and only some of them spawn on a given day, so plan on two trips across the assault rather than one long one.
Since the Night Fae assault is one of the four you have to attend for United Front anyway, this costs you very little extra.