You can only find this free battlestar after unlocking the Retribution Loading Screen.
This is the Retribution Loading Screen for reference:
The Battlestar can be seen on the missile's upper support beam next to the rocket.
Go to the villain lair northeast of Snobby Shores
Enter the mansion through the top where the rocket is.
The Battlestar is close to the top of the missile.
See screenshots and maps below to help find the location of the secret battlestar.
- You can only locate the secret star after you have unlocked the Retribution.
- There is a bug where sometimes the battle star does not show up. If that happens try logging out of the game, then equipping the Retribution in the Locker as the active loading screen, and then trying again to go to the location.