congrats on beating him
for future reference, if you go to C:/users/%userprofile%/saved games/pokemon reborn you can find your save files in there. The current save is named Game.rxdata, the other files are all backup save files. Just rename them so that one of your backups is named Game.rxdata and load the game.
(Alternatively Cass uploaded a shortcut on the reborn discord for getting to the save files folder directly, this can help to get there quickly every time you need to, or just create the shortcut yourself)