Meemitone posted an unlock on Discord: https://pastebin.com/15B4Xz95
Stuntman wasn't even on the team. SS Tachyon, The Sentinels, and Captain Cosmic vs. The Chairman in Pike Industrial. CC used his incap power to let Mainstay use Hurl Into Darkness, finishing The Chairman off and winning the game. Chairman was killed with damage, not the destroy effect. Tachyon was also incapped.
In that light, I tried my own game: Greatest Legacy, FS Parse, The Sentinels, and Dark Visionary vs. The Chairman in Pike Industrial. Mainstay finished off both villains, first The Chairman and then The Operative. The game ended when Parse used her power to let Mainstay use Blackout and take out The Operative. No unlock.