If all stages start with 100% shields and there are more than 50% shields remaining when the final boss is destroyed, the extra boss will appear.(NORMAL or higher only)
We encourage all of you who are skilled to give it a try.
Update to add an extra boss
Source: Steam Community (opens in a new tab)
Build and release context
Steam update times within 24 hours of this post. Matching dates do not confirm which build it describes.
Build 9591606 · public branch
Steam updated · First observed