Luffy was closer to dying in this arc than ever before, because of Magellan. How can you say he's outclassed every threat? Perhaps your complain should more specifically be tailored to "Unfortunately, Luffy has yet to be weaker than, yet not so much that he doesn't stand a fighting chance against, another villain since before Amazon Lily." Although even then, the marines showed up and mopped the floor with the strawhat crew back in the Shabondy Archipelago arc.
Anyway, I don't know how it could be any more ironic for someone to honestly watch a shonen and say, "it sure would be refreshing if there was more of a convenient, tier-bracketed 1v1 battle structure in this arc."