I like to think that Oda has been pretty consistent in showing us that Luffy is pretty damn good against really big enemies.
Against Lucci he had trouble using Gear third, and one of the 2 said something along the lines that it has enormous destructive power, but is kinda slow.
Against giant targets that don't have that much mobility, Luffy can unleash his most destructive attacks and he'll probably hit spot on.
It happened with Moria, happened with that giant during the war, happened with the Pacifista (first time around), and, to a degree, with Noah. And now it's gonna happen again against Caesar.
I think Luffy is really happy when he's facing a giant enemy.
Edit: Also, concerning many of the SH's not having much to do during an arc, it happened before in other arcs; remember that Oda basically wrote Zoro off Drum, and it's not like Usopp did much more than him.
Now, Oda has to juggle with not 5, but 9 crew members. Every once in a while, things like that are bound to happen. Franky goes to the Sunny for some chapters, Zoro goes to train in a frozen lake, Robin and Usopp are there but not doing much. . .Then, next arc, Usopp, Robin and Franky get their awesome moments while Sanji is on the ship recovering from his leg injury and Nami and Chopper look over him. . .
Or something along these lines.