Revising and expanding my above: if I were designing this unlock series (and no, this isn't me admitting I'm MigrantP or anything!), I'd use the existing Prime Wardens as a template. To review, here's PWAA's unlock condition:
Lose a game to Akash’bhuta with the regular Argent Adept on the team. Then, defeat Akash’bhuta with the regular Argent Adept as the first hero; the rest of the team must be regular Haka, regular Captain Cosmic, regular Tempest, and Redeemer Fanatic (in any order). Each of the Argent Adept’s instruments must enter play during the game.
In short, "play out the formation of the Prime Wardens as a team": Akash-bhuta tears Argent Adept a new, ahem, whole note, and in response, he teams up with the rest of the Wardens and pulls out all the stops (so to speak), making sure all of the instruments he has collected enter play.
So what would be the equivalent of "play out the appearance of the Xtreme Prime Wardens"?
Easy first change: they need to lose to Progeny, not Akash'bhuta.
Easy second change: they all need to be the Prime Warden versions, and all five need to be in both fights.
Third change, not so easy: since unlike Akash'bhuta, they didn't go back and beat Progeny, but they had just fought Biomancer, make the refight a team fight including Biomancer (and, as I said above, probably put it in Atlantis, though I'm not willing to insist on that).
Fourth change, and this one is the great unknown: what is the last condition? On Steam, Feral Yoda suggests that AA needs to destroy all his instruments, leaving the Adept with only his voice--which would also fit the early-to-mid-'90s "Dark Age of Comics" feel of the whole endeavor, "I need to destroy these important things!" Alternately, maybe AA needs to injure his teammates, since that's what XPWAA does. So this leaves us with:
Lose a game to Progeny with the five Prime Wardens and Prime Wardens Argent Adept as the first hero. Then, defeat a Villain team including Biomancer with the five Prime Wardens and Prime Wardens Argent Adept as the first hero [in the Ruins of Atlantis]; [each of the Argent Adept’s instruments must be destroyed during the game].
(Brackets on that last clause because it could be something else.)
I haven't been able to try it yet, though.