After spending two weeks leveling up dark magicites, I went back to Siren with an actual Dark team, and this time I managed to master it in 27 seconds.... with only two old darkness relics: Orlandeau's BSB and Shadow's BSB. I'm kind of surprised that actually worked, but I guess a really focused magicite deck works when you put in the effort to max it out. Exact team lineup was:
Shelke -- Wrath, Entrust
Onion -- Wrath, Entrust -- pUSB
Orlandeau -- Dread Weapon, Banishing Strike -- OSB, BSB
Yshtola -- Curada, Shellga -- BSB
Shadow -- r2 and r4 of the dark ninja stuff -- BSB
Magicite:
Main -- Shadow Dragon (inherited lvl 15 empower dark)
Phantom (inherited lvl 15 empower dark)
Isgebind (inherited lvl 8 deadly strikes)
Firemane (inherited lvl 3 dampen blizzard)
Necrophobe (inherited lvl 6 dampen holy)
Video:
Basically I put up Orlandeau's thunder god mode, dispelled Siren's haste/regen then just kept spamming his BSB with entrust batteries feeding him. Worked a lot better than I thought it would. I've now farmed Siren a few times with this setup and it's very reliable (doesn't require dualcasts, though they help get it under 30 seconds).