Where are the Dark Menhirs during the Grandmaster Battle?

Also, invincible monsters can be poisoned, like the mirror ghosts.

 I managed to get a better SS of it

I didn't try poisoning it cuz if I do the party will get pissed at me. (I was just a tagalong on that run so I wasn't supposed to be there anyways, I was only in cuz I had no split.)

I'll try poisoning it when I get invited to a successful run.