Well shit, it actually worked. Took two tries, but I got her with GZL/Athena/Valkyrie/Meimei/Echidna/GZL. I'd take a picture but my phone is broken and can't screencap any more.
First run I got an unfortunate heart skyfall against Hades and had to blow every skill to kill him before he got me. My skill lock resists protected me against Leviathan, I didn't have much trouble with him, then I died to Beorc one turn before my seven-star was back up.
Second run I used the seven-star and a GZL active first-turn against Hades to one-shot him. My skill lock resists worked again, and I entered Beorc with Echidna up and two turns before Meimei was back up. Used Echidna, stalled for the seven-star, and dropped Beorc down to ~20%, then just barely squeaked out killing her the following turn.
That was rough, I could have and probably should have died at a couple of points in both runs. Needless to say, Kali skillmax. That was my first ever Beorc clear, and I jumped straight to mythical.
Next up is Hera-Sowilo, which I know I can clear. Then I'm cleaning up a few of the lower difficulties, then probably Izanami (which won't be easy, but is very possible with Kali and I need both the Pii and a second black Izanami). And if I'm feeling really masochistic I might round things off with Wadatsumi. I may try Athena against him. Now that I have Odin to help mitigate Siren's pre-emptive gravity with his active I think I have a shot.