Screen-sized boss on a Gameboy is hard to pull off. Here's one from Nyghtmare Betrayed,

If u haven't tho, grab your Nyghtmare The Ninth King cart so you'll know the story between the 2 games.

Pre-orders for a physical cart are now open! Click here for more info.

Nyghtmare the Ninth King link: https://elvies.itch.io/nightmare-tnk-gbc

Nyghtmare Betrayed link: https://elvies.itch.io/nightmare-betrayed