It's invincible.
No matter what I throw at it, it doesn't get hurt.
It comfortably sits at about 99% of it's health bar while surrounded by dozens of Yugiri's with attack buff up (and the extra strength against monsters), the 50% def down from hatchlings doesn't seem to faze it, even Hildibrand, the highest attack minion I own, WITH the atk/def buff up doesn't scratch it >.>

I thought that maybe I wasn't supposed to attack it, so I tried focusing on the adds, but only more ended up spawning and with my broken gates I could neither regenerate nor call new minions, so I'm pretty sure I am supposed to destroy the demon brick .-.
No enemy structures to destroy in that battle either.

I never had any trouble with the trials before, but this one just wasted 5 hours of my playtime and I still have no idea how to hurt that thing and I'm so sick and tired of it -.-
Is it only vulnerable to one certain minion or something? Since the fight is losely based on Amdapor, I can't really imagine it, but why else is it impossible to hurt it?