Maybe it was significantly easier than we expect.
I seriously doubt Oda would pit Shanks into extreme diff against a mere squirrel and a rookie like Loki, maybe he is saving extreme diff for a fight he will show us. High diff fights without conclusion were probably against Mihawk so this might have been mid or low diff.
One burst of Shanks' haki and both DFs on Loki are gone so making use of Loki being surprised in that moment, he might have defeated him quite easily.