Hmm, I guess everybody has a different definition but I never thought Valkyria Chronicles was particulary difficult. The hard mode DLC is insanely hard but the game itself, as long as you know how to use the smoke bombs and what each class can and can't do it seems like you can get through most battles within a reasonable number of tries. Compared to other srpg it is actually fairly easy. Try Wild Arms XF on for size.
But that wasn't your question. My obsession isn't so much with bullshit difficulty but with good difficulty.
Difficulty done right is one that gives you the tools to overcome it. Games that are technical and have a "high skill ceiling". As well as intelligent and varied enemy AI. That you can still dominate, given sufficient mastery of the game mechanics. One of the best examples is of course Ninja Gaiden Black, Sigma, Sigma2.
But even there, my hard mode run of NGS1 is stuck on the effin boulder. I beat Doppelganger Ryu, followed by Alma, and all the goddamn cats inbetween, just to hand in the controller at the indiana jones scene. Which I overcame during my normal run but couldnt suffer through again.
A much worse offender is Bayonetta 1. A tremendous game, with a skill ceiling as high as mount everest. But also with gimmickey battles and qte sequences to ruin it all.
I think Bayonetta 2 is the purest example I can think of for pure skill based difficulty. Especially for someone who has played too much NG in his past and tends to focus on not dying instead of chaining combos together..
Another is N+. That game ramped it up until I couldn't keep up anymore. But I know in my heart it is fair, I just suck, and am just not ready to invest the time.
If you watch the speed run of i wanna be the boshy, that guy keeps casually talking while dominating a game that would bring me to tears within the first level. And why can he do that? Because the level design and mechanics are actually thought out well enough to allow for the insane difficulty.
So there, thats my convoluted take.