i doubt how possible it is legit but you can easily edit your save files to give yourself a sword that does insane damage and change your hp to hundreds of thousands.
Viewing post in After defeating giantess I get a black screen.
Well, then of course there's nothing in place for if you win. You can't win that fight, unless you modify the game. At that point, the devs aren't able to consider what you're doing. That's why you get a black screen, you beat an enemy that was specifically designed to not be a win condition. It's a game over.