Congrats : )
Yes Siren is Max skilled.
Got 3 RKs
What team built do you suggest?
Is it possible to zero stone Legend?
Well then!
Team 1: Luci/Luci + RK/RK/RK/Siren
Team 2: Luci/Luci + RK/RK/Siren/Hera or Hades
Valk is still going to be hard. RK allows you to take half damage from Valk Blade (just over 14k damage). Max skill Siren means you SHOULD be able to heal after a morning star if you manage your orbs correctly.
For T1: At the start, MS, heal, MS, plink away until she casts resurrect. The trick now is to damage her until she is about to use Valk Blade (eyeball it). Put up RK, cast MS, heal (to over 14k to take one hit from Valk Blade), MS for the win. It's not 100% safe, but then again, no team really is.
For T2: Similar strat, except now you have a gravity to assist you. By using gravity when she has 992k HP left (which is ~36% health, so after you've damaged her a bit during Valk Blade spam), you can kill her. It's by no means fool proof, but there's just a different safety net here. The other benefit is that Hera and Hades both have higher RCV than RK. The other stats vary across the board, but the higher RCV may help you. Siren won't benefit from the 4x RCV, but bringing a Lillith doesn't seem worth it (even though she'll bring ~2k RCV to the table).
Anyways, that's my take on it. You can do it, but it won't be a walk in the park.
E: And yeah, I forgot about the bind. That bind can fuck you up bad. Remember, Valk is SUPER hard. I think Zeus Dios may be the only dungeon of comparable difficulty? I haven't touched that one yet, ha.