I've sharpened a friend's Shallot a few times. The blade shape makes things a bit frustrating. Like you mentioned, it is difficult to achieve a great edge on this knife.
The first two times I sharpened the Shallot I used a coarse diamond stone from a Smith's sharpening kit I have and worked my way up to fine Arkansas. The results weren't great but the knife was relatively sharp and I was satisfied since it was leaps and bounds over the condition of the blade when the knife was handed to me.
The last time I sharpened the knife I used Spyderco's Sharpmaker, wondering how the blade shape would respond to the sharpening rods. The Sharpmaker only seemed to sharpen parts of the blade because of the unusual shape. After an hour or two, I managed to get a decent edge on the straighter parts of the blade and then used a whetstone to sharpen the spots the Sharpmaker couldn't reach.
