ABOUT COO:
Well, I read a really cool triology called Mistborn… Mild spoiler, there they can burn a metal called 'Atium' which basically lets them see a little into other's people's futures. That makes them literally INVINCIBLE in a fight. As they'd know exactly where other people would dodge (it also ennanced their reflexes). There was something, though. If too people were burning Atium at the same time, they would be changing what they would do with each image of the other person's future, and so the images go on an infinite loop and just become irrelevant clouds around the person. Maybe that is what happens when two COO user's fight. Because, as ilustrated by Enel and his priests, Mantra is fucking overpower. All of them only lost when they lost control of their Mantra. But it isn't that good a thing when you're not the only one having it (it's a radar and nothing else).
That's how I'll see it until future explanation.
About Luffy and Monet.
Let me remember you that Luffy isn't giving two fucks about the kids. He doesn't even want to beat Ceaser. He wants to catch him, because Law said so. That's why he went for the catch, instead of punching him brainless, and only resorted to punching when grabbing failed. So to him Monet is just a barrier. If he was mildly serious, he might have punched her, but he wasn't trying to defeat her. So he threatned before, and I'll assume there's more to her powers than cold. Maybe Yuki Onna Zoan, and her embraces steal a person's life force? Seems interesting. Only downside is that being an Harpy named Monet who uses awesome glasses is already cool enough. Making her a Yuki Onna... I don't know if I can handle it. :<
Still, Luffy did a great job of finding an escape route. Let me remember you that the hut was too small for him to swing an elephant gun. So he'd have to punch it 10 times (1 time broke a single hut)
And let's not downplay Monet. She seems as flawless as Alabasta's Robin.
Chopper protecting the kids was heart wrenching and just plain endearing. Who the hell said there was no character development? I see plenty here...
Monet appearng there was just :X damn her, so evil and yet... how am I suposed not to like her... Her smiling face with the falling snow. The snarky retort. She's the perfect vilain. If she gets defeated this arc I'm gonna be disapointed. She needs to fly away :<, but then her vs all-the-strawhats (-Luffy, Chopper and Franky)+ Samurai... She'd need to be a Beast...
Eager to see Brook's reaction to her power. He is the cold guy, after all.
Law and Vergo... I agree with some explanations posted here.
Vergo was already toying with his heart. Law managed to snatch it, but got soro'd and kicked to the face. Him managing a hit is a proof of how overpowered he is. You need his own ability to defeat him. 8D
Smoker's entrance was just badass. And now I'm eager for the fight. Smokey's fights are awesome. Just love the smoke dynamics, with him swirling around kicks and leaving the jutte behind, throwing some faint's in. He's the logia who better uses the flexibility the power grant's him... (probably because he's the only logia without an axe 'you die if you touch it' ability... Kizaru has the laser beams, Aokiji can just freeze... etc