If you pay attention to the map, you’ll see a single small lake in this area. Just west from this small body of water, you’ll find a small staircase with a couple of pillars, leading you into the mountain, and towards the Archon’s Grove entrance. Proceed through the door into the dungeon.
“Archon’s Grove? Indeed, that’s the site where the legendary poet Nenamil, Archon of Song, discovered and named the Rose-of-Archon, the flower precious to the Goddess Mara. It’s said to be southwest of Shimmerene somewhere.” – Inkeeper Vintenwen
As we go inside the dungeon, we should always keep the the left. As the path gets narrower, we come to the end of a river and the start of a waterfall. At the very edge of this waterfall, on top of a small rock, you’ll find the Archon’s Grove Skyshard location. Don’t miss the opportunity to pan your view upwards, and be amazed by the scenery.
At the mouth of the falls in Archon’s Grove
Sand Kingdom Moons - Super Mario Odyssey Power Moons