^ This.
I follow One Piece regularly since I love the manga, and even post time-skip Luffy is totally incosequential if compared to Lord Boros.
Saitama only used his Serious Series against Boros' most powerful attack, which was enough to destroy the whole surface of Earth; for the rest of the fight, he jobbed.
Lord Boros would insta-kill Luffy.
This is more so a thread for the near future when Saitama starts to fluster or Luffy surpasses him outright
However...
Luffy did heavy damage to Noah which is a vessel half the size of Fishaman island, plus he did it underwater. Reshaping Chinjao's skull was a feat previously only done by Garp who (iirc) shattered 8 mountains in preparation to face him
All three Haki forms, damage soak and rubber flesh can keep him in the fight longer than probably anyone so far, not to mention gear 4th
Hmm? Saitama wishes Luffy was strong enough to slightly fluster him, but it's not happening. Not if Luffy was a hundred times as strong.
The giant guy took out a town in a single wave of his arm, and Saitama casually took him out and was disappointed he didn't hold back more..
All of 'em combined won't stop him from getting one-shotted. Luffy is slow and not near tough enough to handle the shockwave of Saitama's normal hit going through his body, even with armament haki and rubber combined.
He's not near as tough as Boros.
Boros hit Saitama to the Moon, and Saitama jumped back.
He punched Boros's attack, but that could've been something physical enough to be affected by a shockwave. There's a difference between, say, punching fire (physical, easy to affect), and punching light (not so much).
Well in One Piece elements don't matter much considering you can basically hit anything if you've got enough power.(Even Kizaru for example when he fought Raleigh) Now Saitama doesn't have haki, but I'm sure against an OP opponent, or going to OPland, he'd have his own equivalent. It'd be ridiculous to think he didn't.