My vote is still Sans.
Shaggy's Magic Cards are cool and all (even though I can't find anything about them) and his shield is powerful, but Sans's Danmanku pretty much nails all of that. No matter how many things Shaggy summons, they will just step right into a hail of bones. And the Yellow Lantern shield, while powerful, will only be able to feed of Shaggy's fear. And while he may be scared of Sans' if he's using the shield, that means that he is standing and fighting, so he's not completely scared. And the shield will eventually get worn down. The Reality Warping, there is no concrete evidence of Shaggy using that in a combat situation, so it is getting counted as inapplicable. And Sans' teleportation kind of nails any sort of attack that Shaggy can throw, as he can just teleport out of range. (Unless Shaggy uses Zoinksman, in which case, Sans dodges until the mech runs out of ammunition, and continues pounding away at the mech, which will not have the Yellow Lantern shield as Shaggy has never been able to use them simultaniously) And, the argument that Shaggy is as smart as Sans due to Knowledge of Odin, not battle smart, is my answer. Combat intelligance and tactical abilty comes with experiance, and Shaggy never shows any.
I'm sorry. I want Shaggy to win, I desperately do. But I can't see it happening.