Thanks to WotG, it is now possible to get up the ledge by talking to the Lycopodium (NPC) in the past and from then on in the present you can trade an appropriate flower to the sparkling lights to instantly move up the ledge.
You can also use the Atmacite Refiner to teleport to Beaucedine, which puts you near the Batallia zone line.
Spawning them while sneaked will not prevent aggro.
A party will only need to kill the NMs once. Trade the holy water after the NMs are dead to the sarcophagus.
If you zone or die after popping them, the rest of your party can pull one at a time.
It isn't necessary to kill all three NMs. If you pull one NM and then wait for the other two to despawn before killing it, you only need to fight one.
If you are defeated, immediately touching the Sarcophagus will result in "Nothing Happens". Wait a few minutes and when you touch it again the NMs will respawn. You do not need to trade another Holy Water.
Once they have been defeated, re-investigate the same sarcophagus onto which you poured the Holy Water for a cutscene and the Star ring.