So, I thought this would be a good place to bring up a really cool set I've been using for a while-
Shiftry
Leftovers
Trait: Chlorophyll/Early Bird
EVs: 252 Atk / 252 Spe / 4 Def
Adamant Nature
-Substitute
-Swords Dance
-Sucker Punch
-Seed Bomb
Basically, this set tries to abuse Shiftry's surprisingly awesome defensive typing to its fullest extent. When you think of physical walls/cockblocks in NU, you'll probably come up with stuff like Alomomola, Musharna, and Misdreavus. Shiftry's typing, however, gives him a handy resistance to every move that those three use, allowing him to set up comfortable on all of them- Missy's Shadow ball and Alomomola's Waterfall both take 2 hits to break his Sub, while Mola's Scald takes 3 and Musharna's Psychic does nothing. In addition, since Substitute on Shiftry is so rare (it's on ~11% of all sets), most of them will have the courtesy to give you a free turn by attempting to hit you with the appropriate Status. From there, you need only boost to +2 as they either poke at your sub or switch out to another pokemon, and start wrecking their team.
The real beauty of this set, though, is that most teams don't carry a solid physically defensive mon other than the above three, which means that their only way of checking Shiftry is through offensive pressure. The catch is, though, that the number of offensive mons which can take two consecutive sucker punches (or a single +2 sucker punch) is remarkably low. Primeape, for instance, has a 50% chance to be OHKOd by a +2 Sucker Punch after Stealth Rock, even without factoring any defense drops from Close Combat. Once set up- which, due to the prevalence of the aforementioned walls, is pretty easy- this set almost guarantees a kill, and can easily punch holes in an opponent's team for it's allies to sweep. Mons like Sawk in particular will appreciate having a physically inclined ally which can break through the walls which give it trouble, while it in turn can eliminate mons like Weezing or Mandibuzz which wall Shiftry to no end.
On that note, this Shiftry, like all Shiftry, gets screwed over by Mandibuzz, and the loss of Nature Power can mean difficulty with Skuntank, although if it runs Crunch over Poison Jab or you're at high health with a Sub or Swords Dance, you might have a chance. Unlike standard Shiftry, who's a sweeper in his own right, this Shiftry excels at punching through a few specific (and irritating) defensive threats, and won't
usually sweep all on his lonesome.
So, yeah, Sub Shiftry is awesome and you should give him a try.