Is Diablos hard???

[Heath]

Can't you just blind him? And then all he can do is hit you with demi, which can never actually kill you

ok... but do you know any enemys that happen to have blind cuz all i have right now is cure

I think you may have missed your chance to get it. You can get Blind either in Dollet (though I don't know if you can access it on the Dollet mission) or off the Grenaldo boss, but if you haven't gotten it already then I don't think you'll be able to get it for a while (though still on Disc 1).

However, as others have said, if you keep your health up, cast demi on him until it's ineffective and then just finish him off as you normally then you should be okay. The sooner you do it the easier it'll be and the more use Diablos will prove to be!