Copycat Clefable (Shell Smash Clefable)
Clefable @ Life Orb / White Herb
Ability: Magic Guard
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
-Copycat
-Moonblast
-Flamethrower
-Thunderbolt
Originally saw aim and some others using this. This set is somewhat gimmicky but it's fun as hell.
Here's how it works: you pair it with a Shell Smash Torkoal, or Blastoise, whichever you prefer, that's holding an Eject Pack. It smashes and gets switched out immediately. Then you bring in Clefable and copycat the Shell Smash. Now you have a Clefable with +2 Special Attack and Speed, enabling it to kill almost everything in the tier one one hit while outrunning Zeraora and Dragapult. Life Orb for insane damage output or White Herb to fix the lowered defenses, since an uninvested Clefable at -1 -1 is very squishy. It still tanks Conk's Mach Punch though.
It's very important to note that Copycat copies the last move used in battle, not your last used move. This means if you bring in Clefable against something that's faster then Clefable will copy their move instead instead of Shell Smash. So be careful with that.
This set is really effective against people who aren't familiar with it. If you see someone click Shell Smash with Torkoal / Blastoise and immediately get switched out for it you'd probably wonder what the hell is going on too. Then you get 6-0d by Shell Smash Clefable.
PSA: Other mons that get copycat, whether they're viable or legal in OU, include the Mr. Mime line, all Eeveelutions, Sudowoodo, Lucario, Mimikyu, Marshadow, Dubwool, Liepard, and Slurpuff.