Oda is long past the point of writing Saint Shanks in a passable way, this dude is a Gary Stu among Gary Stu’s.
But it’s okay. I am 99% positive Saint Shanks didn’t defeat Loki at all. I’m pretty positive it was actually Imu who beat Loki, and Saint Shanks just chained Loki up so he wouldn’t challenge Imu again. Saint Shanks got the credit because no one knew Imu was involved.