That grandpa taught the wrong first lessons; it's "Never point your weapon at anything you don't intend to destroy", and "Always assume that your gun is loaded at all times", and "Keep your finger off the trigger at all times until you surely intend to fire."
calling it now, since big bad revived kagura, during her dying breath she's going to revive raito and then, kagura, with the power to choose now, will undo yomification and then have many kids with raito
Even Brittle Iron Tempered by Hellfire for 2000 years becomes Unbreakable
But for a lot of that he probably wasn't the strongest around, he just survived the longest because the actual strongest ones got too ahead of themselves and were eviscerated by the two gods.