I sadly can't remember when they changed it in the JP version, but they removed the requirement of participating in the Arms Phase in order for you to unlock the Elder battle. Now you can sit out the Arms Phase and just do Elder only. (Probably still need the ship to defeat enough Arms for Elder to appear.)
I would imagine this being the exact same for the NA version, hearing stories that they are using the latest/recent PSO2 Client and all?
Sadly not owning a XBOX One myself and therefor unable to test it, but if anyone is willing to sacrifice his/her Elder UQ for the greater good to look into this matter. Please be my guest, be my eyes and ears goose.
(Perhaps it has something to do with Omega Elder. I can't remember.)