Quick question, I know one method of custom bosses is multiple screens, are their any others.
Custom Bosses
Not really. Technically, you could... actually, not. So, multiple screens really is the only way. 😉
multiple screens?
what does that mean?
It means you make the screen a 'enemies->secret' room. Then, when all enemies (probably fires underneath a custom set of tiles) are destroyed, the 'secret' or really a warp, is activated and it sends you to part 2 of the custom boss battle. Mega Man: Dr. Wily's Revenge DX is a GREAT example of this.
The first boss is a custom one. After you kill all the fires, you are warped to another room where the boss' "hit points" bar at the side decreases, as well as adding more fires/traps to the rooms.