I always read this, but the last valkyrie was the easiest for me. Just had to remember how I did to avoid the attacks of the previous ones, and I killed her on the first or second try. Of course, it was only in normal difficulty (or hard? don't remember), but anyway, I had a lot more trouble with some other valkyries. The last one was a lot of fun though, can't deny that!
The most troublesome to me was the very first one.
Mostly because I was so weak (I think I was still level 2) that I couldn't let it touch me at all.
Then I switched gear and beat her on the first try, lol.
The last one was also pretty hard to me, mostly because she has some much hp, I would eventually fuck up some dodge and get owned by her decent attack, that was either instakill or pretty much it.
I played the game on normal.
The hardest thing was one of the rift fights earlier on, tho.
As soon as I could explore and went and did everything possible, including fighting two level 6 werewolves at the same time still being level 1.
Every touch was instakill and they are really damn fast, but I refused to let it go because if the game allowed me to fight them so I should be able to beat them... why do I do it to myself?