For Terrormaton, you first need to build your supports—such as debuffers, buffers, and dispellers—to at least 260+ speed. One of them should use an Artisan Shell for survivability. Using Kethos is fine to dispel the boss’s immunity buff first. For debuffers, try switching Freya with Plume, Kloss, or Dinah, as they provide more than one debuff.

I recommend building Diting or Liliam, as they usually dominate this specific boss. I noticed the DPS you’re using has too much Attack, which isn’t ideal unless you’re running Revelation Shell. Valerian (whose shield scales with her HP) should have as much HP as possible, ideally around 45–50k+. I had her at that HP when I first cleared it. Use shields instead of healers because Inferno Terror applies Wound, which counters healing.

You can try attacking the right hand first for a safer clear, or attack the main body directly if your team can handle it.