Tip: Make sure that ALL exit-enabled pipes, doors, etc. have a screen exit defined, or they'll take you to Level 0, the "endless" bonus game. Unless Start+Select is enabled, the only way out of it is to reset the game.
A Kirby boss. It will move between the left and right edges of the screen in various different ways, such as walking or jumping. It can also shoot fire. Mario must jump on it to damage it. When it dies, it will fall down.
The sprite can once again end the level. The original sprite modified the SubOffScreen to end level if it despawns. Frankly, that's a little dodgy, but I made it mimic the same functionality (except outside of SubOffScreen so it can still be a shared routine).
Quite a neat boss sprite, however, I'd say it's too easy on its own. I recommend adding more obstacles around it.
Converted to PIXI and added SA-1 compatibility. I made it quite a bit easier to customize and rewrote the RNG routine so it produces better numbers. The old one could be easily manipulated (Mario's position was essentially half the seed) and got stuck into flipping between two numbers unless it was externally seeded properly.