I know that there is a counter for the amount of kills on the stats menu. I know how to open "file0" to hack my location and items. All I need to know is where in "file0" is the kill counter to change a property to (insert amount of kills needed to have Sans ask if I want a bad time here), the codes for the items needed (I already hacked the real knife into the game), and the room code for Sans' bossfight.
(I'm not very good at making "complete" questions yet so edit if you think something I said was unclear)
Best Answer
If you just want to practice, beat, or just experience the fight without playing the game, there are solutions other than editing your save file. You can find a recreation of the fight here and play it at your leisure. There are also several videos depicting the fight on YouTube.
The actual process of getting a save file is very complicated because of the various checks to make sure the game data is logically consistent, so it's honestly not worth it. Playing the game and grinding through fights is so simple during a genocide run that it would take less time to get to the fight properly than to learn how to edit the files without corrupting anything.