Spoilers - yes I know game is ancient, but there's still some folks out there that missed out.
There seems to be a unique spell used from the True Earth Rune that we don't have access to, not even Sasarai himself possibly.
It seems to grant The Wind Rune Incarnation the ability to cast a reflect type spell.
I say this because you see it use the True Earth Rune and any spells cast after is reflected back. Maybe I'm wrong? I have been wondering this for years. I checked all the True runes it has access to, and Silent Lake is the closest thing but it blocks magic rather than reflect it.
Edit: This seems to be an exclusively stronger version of Canopy Defense. For the player, it's for one spell. For the boss, seems to be several rounds.