I have heard a million+ times that the only thing that makes the Demyx battle hard is his water clones.
To that I say, fight him at LVL 1 Critical mode and try to say that again. Hoo boy...
Took me almost a full day, to beat him. And no it was not his water clones, as in FM you have 15 seconds to beat 10 clones.
No, it was the hard hitting Demyx that kept on killing me.
But I still wouldn't say he was the hardest Org. battle in a low lvl run. Roxas took me just as long to beat, and at that point I was lvl 28. (I hit a lvl 1 playthrough wall at Jafar & the Experiment)
I even took down both Xaldin and Xigbar, whom I have seen on this thread mentioned multiple times as the hardest, without dying once.
So Roxas is the toughest Story battle.
As for data battles, I'm not quite sure. At lvl 99 I don't die 99.6% of the time, and when I do it's because I screwed up.
I had the hardest time on Vexen back in the day, but once I beat him,I never had troubles again.
The thing is that there is a trick to all the data battles, that makes them all "easy"
Now the only battle I tend to fear is Luxords, and that is because of his "Get four circle commands" minigame. Unless you know how to loop him endlessly before he goes critical, there is no avoiding it.
So to me the hardest data battle would be Luxord.