This game mode does not have any official mode rules, due to it being unreleased. However, the name of the mode gives us a hint that it might've been that a group of players needed to defeat a powerful boss.
This is the only mode that hasn't been released.
It was also replaced by a different mode, which is Flag Capture.
Despite it stating an "Under Development" sign, the Pixel Gun Company didn't release the mode.
The Pixel Gun Company probably decided to cancel the development of this mode.
Siege also had a similar "Under Development" sign, but Siege was then added into the game, while this was not.
In 16.2.0, a mode similar to a Boss Battle was added: Raid.