Switch to SAB/SEN and use your debuffs. When you see it use Looming Wrath and when you're done casting your debuffs, switch to RAV/COM and keep attacking until Hecatoncheir starts attacking you. Hopefully, Fang should be getting attacked since Looming Wrath targets to the person closest to it. When this happens, switch to MED/SEN and wait for it to use Looming Wrath again. Repeat until win.
If your health is okay, you can switch to SAB/SEN and manually enter debuffs to raise the Gestault Gauge. Good luck to ya.