We should be aware by now, that Luffy's fights are of 2 types. The first one is when he just fights for the sake of fighting (He is not really motivated). And the second is when his friends or people he considers close are threatened. (Robin, Vivi, Ace etc.)
And there is a huge gap is luffy's strength in both types. So far its been a 1st category duel. But I suggest you hold on tight, because the moment Law's life will be threatened or some other news comes floating in, It is going to turn into a 2nd category battle.