The recent episodes revealed a lot more about how Law use his ability to do the things he did. If what's seen in the anime were to act as a supplement to canon manga, then I think I have a good idea of how his ability works and probably a way to overcome it.
As an example: the anime revealed that he (Law) had to deploy Room over the four StrawHats before he does the stabbing action with his nodachi (aimed at them), pulling out their hearts literally and Shambles it up before returning the swapped hearts into their bodies. Correct me if wrong but I don't recall seeing this being stated in the manga. This was what prompted an idea over his ability.
Below are my personal analysis of his techniques:
Deployment of Room and its sphere of influence
In order to utilise his Ope Ope ability, he needs Room to be deployed over the battlefield. After which, the whole ground belongs to him as an operating table to a surgeon. His attack moves, like his slashes, is only effective (in the direction) within the radius of his Room. As Smoker mentioned, if one gets caught in the direction of his slash (inside of his Room sphere), then despite the nodachi is not in range, they will still get cut. That being said, if you recall the scene in manga of Law cutting Vergo, the SAD tank and the mountain, I think Law had the whole island enveloped in his Room (or at least enough to cover the mountain).
The invisible slash and the cut
At the slashing swing of his nodachi, the direction of the attack extends all the way until the end of his Room and anything along the way gets cut. This can be negated if the opponent has enough level of haki to withstand it. When Law was about to cut Tashigi for the second time, after the epic quote "Losers don't get to choose the way they die", Smoker was able to stop his nodachi from cutting her. I think Smoker imbued enough haki in his jutte to combat Law's power at that point in time. But if Law were to fully concentrate and summon a large level of his ability (perhaps plus his own haki level in it as well), he could overcome the level of haki armor - this is referring to Law when he cuts Vergo in his full haki imbued form. We knew Vergo was quite a significant haki user, but you can just feel the momentum as Law said the things he said before he unleash the epic slash that cuts Vergo, the SAD tank and as well as the mountain behind it.
The nodachi (or any bladed weapon he got his hands on)
I observed that he uses the nodachi for the invisible cuts (well duh!) as well as when he Shambles the hearts of the StrawHats (he did the stabbing action at their direction). In my opinion, without his nodachi or a sharp bladed weapon in his hands, he will not be able to pull that act. In an operating room / table, the knife is an important tool to initiate the operating process, so a surgeon without a knife will not be able to do anything. This is the basis of that opinion of mine
Bare handed techniques
A few technique of his does not require the use of a bladed weapon, like Takt, Mes and Counter-Shock. Takt is to lift any objects in his Room, Mes is to puncture the heart out of a person (I don't agree that this move does not need to be under the influence of Room as stated in article, because the underlying idea that a heart is forcefully taken out from a body) and Counter-Shock to electrocute the opponent (may or may not require Room).
So, after all that, in order to defeat him, my opinion are as below:
1) Try not to fight in his Room in order to eliminate his advantages. Perhaps a really agile fighter like Luffy in Gear Second can quickly go in, do his stuffs, and back out before Law can react - that is if Law was not as fast as his opponent. Or perhaps a fast enough Gomu Gomu no Jet Bullet / Pistol performed from outside of Law's Room - again that is if Law could not keep up with Gear Second's speed. The idea here is to not fight in Law's Room, so Law's advantage of Room will be negated.
2) If had to fight within Room, then always (haki) armored up and focus all attention at Law and what he's doing. The main reason for Smoker's defeat to Law via Mes was because Law was able to distract Smoker's view while he (Smoker) insisted to thrust his jutte through the obstacle, not able to clearly see what Law's was doing. As mentioned above, enough level of haki will be able to negate Law's cuts and invisible slashes. Thus a high level (or at least higher than Law's) of Kenbunshoku Haki for observation and Bushoshoku Haki for armor is required to "dance" with him in his Room.
3) Remove his nodachi or any of his bladed weapon. As stated above the importance of a knife to a surgeon, Law should be void of many of his advantages / attack techniques if he does not have his nodachi in his hands. Reduce him to bare-fist and the fight will tilt to his disadvantage.
4) I have mentioned that Law has a few bare-handed techniques of his own. Just be careful and attack from a distance away from him to and the chances of winning is high.
Agree? Disagree? Plausible? Bullshit? Feel free to share your mind but politely please.