Well I just finished the missions that have fiend Ryu on them, I believe that there are seven in total (feel free to correct me if I'm wrong).

Here's the thing though, I beat all but one of them on my first try, considering how hard it was on the game in story to beat him. A lot of people here probably did too, but I have to brag somewhere, nobody I know plays this game, so they wouldn't know what I'm talking about, hence, they wouldn't care. So I'll share what approaches I used for those who may run into a snag here.

One thing I did, I equipped the Armlet of Tranquility(it slowly restores health) and I used either the Kitetsu, Dragon or Dark Dragon.

The Dark Dragon allowed me to dispatch the two fiend Ryus in one try with a simple method. Fiend Ryu always comes after you, so no need to lure him, then when he's close, use the Divine Cicada Slash on him, he falls for it two out of three times and it takes a big chunk off of his health, but this is for the two FRs.

I'll post more on this if anybody asks any questions, if anybody is actually having trouble with these parts, but I think most could probably do it easily.