Mesprit @ Light Clay
Ability: Levitate
EVs: 252 HP / 4 Atk / 252 Spe
Jolly Nature
- U-turn
- Reflect
- Light Screen
- Healing Wish
Lots of tough competition because Mesprit is so versatile, but here's my offering.
Mesprit is a viable Dual Screener for several reasons. I'm specifically comparing it to Meowstic because Meowstic is the main competition for the role:
First, Mesprit can U-turn. This allows you to fake a scarf/other set early on and use it to pivot with its fast(ish) speed throughout the game. It also allows for more effective pivots out to threatening mons after setting up screens instead of switching blindly or sacrificing the screener 'for the greater good'. Secondly, Mesprit gets Healing Wish, which can be used later in the game after setting up one or two screens for a lategame sweep. Didn't complete the sweep with double dancing Kingler off your first set? Never fear, Healing Wish is here! Third, Mesprit has significantly better overall bulk than Meowstic, which increases the likelihood that it will get up multiple screens throughout the game.
The set is pretty self-explanatory - 252 HP so you have bulk and can decide which screen to set up first depending on the anticipated switch-in or situation, max speed Jolly so you can get screens up quickly (functionally equivalent to Meowstic most of the time unless you're staring down a Scarf). I know you're not allowed to put slashes in these sets, but you could also run Trick Room instead of one or the other screen if you want to bring in, say, Eggy-A to cause some destruction (also, nobody would expect it after seeing a single screen).
A common/basic scenario where this set puts in some work would be:
Switch Mesprit in on something passive/non-threatening (Rocks setters, other Mesprits, etc)
Opponent switches in Skuntank to Pursuit trap. On this turn, Mesprit may set up a screen of your choice (depending on anticipated switch-in)
Mesprit outspeeds Skuntank and sets up reflect (or light screen if you set up reflect last turn)
Skuntank does nothing with Pursuit
Calc:
252+ Atk Skuntank Pursuit vs. 252 HP / 0 Def Mesprit through Reflect: 55-66 (15.1 - 18.1%) -- possible 6HKO
Mesprit sets up other screen or u-turns out into threatening mon to begin mayhem.