@Coruscation:
I'm fairly confident that Law used his own Haki to nullify Tashigi's, since his is much stronger. It fits the best with the general "the man makes the ability" mold of the series. If Tashigi was facing some weakling with Law's fruit, her Haki should be strong enough to deal with it. It should be the fact that Law is Law and as powerful as he is that overpowered her, rather than her Haki being insufficient period.
My impression is that some abilities of a df can be easier blocked by haki than others.
For example, it seems that Law's "shambles" only works on targets with no or low haki (Tashigi), or on people who don't see it coming (Sanji & the others).
It would explain why Law didn't immediatly "shamble" Smoker at first sight - which would have saved him the trouble of actually engaging in real combat. This tells me that he had to catch Smoker off-guard in order to use the body swap, with a strong "single target" attack: MES.
After Smoker passed out he could basically freely use shambles without any resistance.
This theory would also explain why Hancock uses 2 abilities which do basically the same thing:
1. The mero mero beam
2. Slave arrow
The mero mero beam could function as the widespread attack, which basically can't miss due to its range. Downside would be that it is only usable on fodder and targets who don't posses haki.
The slave arrow's have quite the range as well, tho they actually have to hit the target. However, they could have an impact on haki users, maybe even piercing the invisible armor. Thats what arrows are supposed to do, right?
For the hardcore haki users both attacks might not be suffcient enough and she needs to engage in direct combat to petrify, namely with parfume femur. On absolute masters (Rayleigh/Shanks) the whole DF might cease to work.
I think this makes a lot of sense.