Byakuren and the Succubus Castle (RJ374869) : need help with opening Murasa's gate

[#unknownboi]

Hello, I'm trying to grind for 100% state of the game, but currently I'm stuck at this gate to fight with Murasa because I can't find the switch to open it. Hope that someone played the game can tell me where.
I'll upload the 100% save file once I can be able to reach it if anyone interested

 

[DarkFire1004]

Post in the appropriate thread. Thread locked.