If 504 had a boss fight, first it would be absolutely chaotic. Second there would be two very odd endings to it.

It would probably be in phases each one getting more chaotic as it went on. It would probably be split into three of these phases.

Phase one- this would be the most calm of the three,504 not trying very hard yet still destroying you almost all the time. He’s probably use a series of basic bone attacks just to keep you away. As it went on though he’d start getting a little more annoyed with you. This would lead into phase two.

Phase two- this one would be a upstep from phase one by a lot, instead of just bone attacks he’d probably use a lot of magic, while healing himself if left un attacked for more that two turns. If you manage to get get him down to 2-1 hp. It would trigger phase three to start.

( please note 504 would not allow you to use mercy in the two previous phases, in this one that option has opened up)

Phase three- basically if all the undertale boss fights fused times ten, 504 would remove all abilities to heal yourself by taking out the items button. It’s basically phase one and two combined but with physics involved and more extreme.

Endings- there are two possibilities of endings, and neither involve you directly killing 504.

Ending 1- you die during the fight or at the end, probably the easiest yo achieve. You just have to die at any point during the battle

Ending 2- you spare 504, in this ending you would use an ability that comes with mercy. Reasoning, to achieve this you would use the ability reason this would cause a menu To appears with a bunch of possible things to get 504 to calm down. You’d have to use all of the options except “ mention family” and save that till the very end. If you do it correctly 504 who is physically and mentally exhausted collapses to the ground apologizing to you. This is the good ending.

https://m.youtube.com/watch?v=QkFzyVusomk