Uh, encore is usually the first move for each opponent of Shuckle? They sub, they're screwed over a bit.
About the ANYTHING taking it one-on-one... other than water attacks and poison/steel types, which I touched already, nothing'll do enough damage to a Shuckle with rest before toxic kills them off.
So oh joy, I guess EVERY single team'll have to carry a sTalker just to survive shuckle in NU. If they try to set up on Shuckle, they get encored and the shuckle player gets a free switch to whatever will rip that pokemon apart. If they sub, same thing. If they attack, Shuckle can either stay in and take the hits, or encore and switch to something resistant. Anything I missed?
Oh, and I doubt whatever sleeptalker it is can do enough damage to this Shuckle, unless it's a water type.
Shuckle@Leftovers
Careful +SD -SA
252 HP, 6 DEF, 252 SD
244 HP, 497 DEF, 614 SD
Toxic
Encore
Defense Curl
Rest
Enjoy, the shuckle I use. Defense Curl will cut off all physical attacks, 614 SD is highest possible naturally. Pelipper, the strongest STAB surf in NU, does 39%-45%, meaning anything weaker does less.
Why are you so eager to get Shuckle to NU anyways?
EDIT: Thinking about it for even longer, if the sTalker were put to sleep, fat chance of that happening again. And if it was self-induced by rest, Encore = screwed.