You will need two free apps from the Google Play Store:
battle makers on Android are mobile applications that allow users to play, create, and share custom boss fights inspired by the game . These apps typically replicate the "bullet hell" and turn-based combat mechanics of the original game, providing tools for fans to design their own attack patterns and character encounters without needing advanced programming skills. Popular Android Apps undertale battle maker android
If you want to create a battle on Android right now, the content you are looking for falls into two categories: If player y-position > threshold then spawn downward