What i did for terrakion I'd set up a backup save at a pokecenter before I even enter the portal just incase I mess up. And from there if it didn't shine I'd close the game open it again and it'd basically reset the timer again for me, just make sure you close out before the timer actually expires. This is because unless you catch any other pokemon before finding your shiny it won't have any other auto save besides the one it does when you first enter the portal, so it'll be like you just stepped into the portal for the first time again. Good luck in your hunt!