This isn't much of a "challenge" mode -- it's one extra scion in play, yes, but other than that the game runs pretty much as usual. Voss is so difficult to damage normally that I rarely have much or any damage on him by the time he flips.
I followed my usual strategy -- split up the party, finish as many missions as possible, and take out Oblivaeon's shield with the counter on "1". That tactic tends to work really well on Primary Target since there's really no randomness to this shield and it's based on missions to begin with. I ran through a LOT of heroes in rounds 3-5, though -- it's a rough one for a few minutes there, I admit.
I can't fight oblivaeon and get a mint, though -- I need my dang rewind, there's too much going on and I always miss crucial things or just mis-click somewhere along the line. I won it first time with an environment still in the bank, but I did have to bust out the 'wait, back up a second, what happened?' several times.
I managed to burn through three of Legacy's four forms, but to good effect -- just after getting Oblivaeon into his second form, Legacy tanked most of Borr's blast using a Heroic Interception, and that blast killed two additional scions AND flipped like three missions all at once, at the low cost of one Legacy, which made the whole game a lot simpler to deal with; and then later another variant Legacy used a second Interception combined with Next Evolution to entirely dodge the 20 infernal from San Alonzo going down (which also took down Empyreon's flipped side).
I dunno, it didn't seem that rough to me, other than that one turn where I lost almost the entire hero list because I had three scions ganging up.
I think there were only two or three rounds where I acutally left a scion unattended to target more damage on Oblivaeon (or Voss), and one of those was the last round anyway.
Unity was probably my MVP -- I ended up with Unity running the Mecha-Knight with Champion Bot and Legacy's Inspiring Presence, so it was doing ridiculous damage. (Note that if you have Idealist in play, you can manipulate the cards under Mecha-Knight without actually having it in Idealist's play area, which can be important -- you can power it up, but you wanna have Superego running cuz Mecha Knight MUST hit the full number of targets per use, even if that means hitting allies.)