Although shells are hard to come by, in my personal opinion Scorpion Rings are the way to go. I did scorpion arrowheads I believe until 53. And then from there I moved right into rings until 60.
My reccomendations:
Scorpion Arrowheads to 53 (Wind Crystal : Bone Chip, Scorpion Claw)
Blood Stone to 57 (Wind Crystal: Giant Femur, Grass thread, Fiend Blood.)
or
Beetle Knife to 59 ( A little pricey ) (Wind Crystal: Beetle Jaw, Oak Lumber)
or
Scorpion Ring to 60 (This is what I did.) (Wind crystal: Scorpion Shell)
(( If you're having a hard time completing all the levels.. Stop at 58 and you can start making Demon Arrowheads. they sell for about the cost of materials. But breaks hurt. These cap at 63. ))
Now the best part about scorpion rings is that when they are desythed successfully. You get your Scorpion Shell back. They npc for a big loss, and they sell for crap on AH. So my opinion is just desynth and reuse. Rinse and repeat until you hit 60.