So I have to restore a few stonehenges before fighting Nuadha. After restoring two, I got "sealed" one (there's some red chain wrapped around it) and the game tells me to defeat Aonbharrs near a floating stone to unseal it. However I've already killed like 20 waves and they still just keep on respawning. Anyone can tell me what am I missing? Sorry for writing in a rush, I tried to be as coherent as possible.
EDIT
Finally got it. It turned out that there were two sealed archs next to one another and I went to the wrong one^^"