FearLess Cheat Engine

Hello. I've seen enough videos on youtube from Dark Souls, Elden Ring and other games where boss fights are created... I want to try to do it myself, in any interesting games. I'm a beginner... Tell me, is this difficult to do, for example, in old NES games? I'm fiddling around with Mega Man 6, going through the Mega Man and boss health data structure, changing different values, sometimes there are changes in the game, sprites change to boss sprites... but it's all too hard to do right. Tell me, is it possible to somehow remove the player character and add a second boss instead, even with glitches? Or other ways? I'm just curious to try it...

if game have source code or IDB then try to do something with code (not in cheat engine of course, if you want to make it in Cheat Engine it will be harder)