For the record "hex editing" is not a concept all that meaningful to hackers. You might as well be asking "anybody here good at typing" when what you actually want is someone that knows Tagalog. Indeed hex editors are considered very basic and crude tools, and while they allow you to edit any part of the file and thus any part of the game it is not a good way of doing it.
Anyway this might be the sort of thing you can do with a cheat -- not all games will and it might be more complex but the basic thing a programmer would do is save a flag to say you have fought the boss. You find where this flag is and force it back to not fought and you can then go endlessly.
Alternatively if you can figure out how to edit maps you could place the final boss in like any standard enemy, or if the boss takes place on a separate map/level copy and paste that level over another one (say another random small dungeon and depending upon how it is coded it might allow you to go endlessly there.