Moves
Endeavor and Counter are what barely make this set more than a gimmick, allowing Cleffa to completely incapacitate a wall after it has taken a strong attack. Endeavor also allows Cleffa to remove all non-Ghost-types or Substitute-using special sweepers when paired with Thunder Wave. Thunder Wave is useful for punishing switch-ins and allowing Cleffa to rely less on prediction when choosing to use Endeavor or Encore. Encore is extremely useful for crippling the few sweepers that set up on Cleffa, notably users of Substitute that otherwise completely shut down the set. Encore also allows bulkier sweepers, such as Bulk Up Timburr, to be managed as well as serving to give Cleffa something proactive to do against these Pokemon, and allows Cleffa to not lose all momentum.
Set Details
Magic Guard prevents Cleffa from prematurely breaking its Focus Sash—all but guaranteeing a retaliation from Cleffa. The HP investment allows Cleffa to absorb the largest amount of damage possible in order to double it back with Counter. The lowered Defense IVs and a Hasty nature means that even resisted attacks do considerable amount, perfect for powering up Counter, while the lowered Special Defense IVs allow Cleffa to lose more HP for Endeavor. The Speed EVs allow Cleffa to hit 12 Speed, outspeeding Download Porygon, Snubbull, and all slower walls, allowing it to get off an Endeavor or Encore on these targets.
Usage Tips
The set revolves around taking advantage of Counter and Endeavor to effectively cripple or outright KO a majority of the tier. Little Cup is composed predominantly by physical attackers, allowing Cleffa to match up comfortably versus a large percentage of the tier. When using Cleffa, be sure to preserve its Focus Sash by never switching into any attacks with it. Know what your targets are, and if these Pokemon are mixed attackers. In the case of either an either special or mixed attacker, Thunder Wave is the safest option to click, and should be followed up with Endeavor. Encore should be used to manage bulkier sweepers, such as Bulk Up Timburr, and help maintain momentum versus Pokemon setting up Substitute. However, Cleffa does need to be wary when dealing with the likes of Shellder, Rock Blast Omanyte, Honedge, and Snivy running Substitute, as all of these sweepers are largely indifferent to Cleffa's presence.
Team Options
Cleffa has the best synergy with Pokemon that are powerful attackers but can be setup bait for intimidating sweepers, especially those that would require a backup plan, such as Choice Scarf Bunnelby, Stunky, and Archen. Teddiursa, Mienfoo, and Snover also pair well with Cleffa due to how often these give up free turns or are just too weak to prevent opposing Pokemon from settin up, and thus give Cleffa reason to be on the team. Additionally, Cleffa can completely cripple otherwise solid answers to these Pokemon with Endeavor. Stunky, Munchlax, and other Pursuit trappers trap and remove Gastly, producing a reduced opportunity cost for using Cleffa and again offset Cleffa's inability to handle Ghost-types. Pokemon with pivoting moves—Mienfoo, Elekid, and Deerling in particular—can help lure in slower walls for Cleffa to take advantage of after its Focus Sash is broken. Fletchling has a tendency to give Pawniard and most Shell Smash users free turns to set up, meaning Cleffa is rather helpful for subduing these opponents and potentially Endeavoring down a wall for Fletchling; Fletchling also can pivot Cleffa into slower bulkier walls while also revenge killing the strongest Cleffa counters. Cleffa's Endeavor has notably great synergy with Snover, Amaura, and Hippopotas, KOing the foe in tandem with the weather effects while Cleffa avoids damage. In certain cases a powerful attacker—such as Fletchling, Corphish, Gastly, and Ponyta—might be required to follow up Cleffa to remove a particularly troublesome setup sweeper, though generally these Pokemon have greater synergy with Cleffa outside of just this use.