I think Luffy is most likely defeating her if the intent is serious enough since Haki should be able to tackle stoning and if the Law issues with affecting people wielding a strong enough haki also works with her Mero Mero that's a double factor, yet Luffy could still most likely blitz her nevertheless, conversely. But I concur about the former. There's too little evidence of Coby standing that elevated.
That things about Law DF and haki is a mess. How is that work? Yonkos are haki monsters so nothing happen to them, but what about those attack Law used against Kaido? Gamma Knife, Injectin Shot(not sure but I think it was those)... How they hurt Kaido if Law's haki isn't strong enough?
And what about Law's fight against Smoker, Vergo, Doffy? He needed to take Smoker off guard to proper use his DF against him. Against Vergo full body haki he used without a problem, against Doffy he made Doffy bleed when Doffy hold his sword while coating his hand on haki.
I can understand he using his ability on Luffy and Zoro because they "let", they didn't try to protect themselves with haki against Law since they were a team. But those others fights makes no sense so far.