Moves
Follow Me lets Clefairy redirect and tank hits for its partner. Snatch is an excellent support move, stealing Tailwind, Wide Guard, and Quick Guard for Clefairy's more offensive teammates. Protect lets Clefairy stick around for a bit longer, allowing your other two Pokemon to further take advantage of Friend Guard. The last slot can be somewhat tricky to choose, as Clefairy has plenty of viable options. Heal Pulse allows Clefairy to give any of its teammates a second chance at life, which is especially deadly when paired with Friend Guard's defense-boosting power and Trick Room, under which Clefairy suddenly becomes a speed demon. Helping Hand lets Clefairy boost a partner's offenses and makes up somewhat for its bad offenses, and it is especially useful if your team is full of spread move users. Icy Wind is rather weak, but it gives non-Trick Room based teams another form of speed control, while Moonblast is Clefairy's strongest and most reliable STAB attack.
Set Details
Friend Guard is the main reason to use Clefairy over Amoonguss or Togekiss. The ability reduces the damage that Clefairy's partner Pokemon receive, making them very hard to deal with. 252 HP EVs and 172 Defense EVs let Clefairy always survive Adamant maximum Attack Mega Kangaskhan's Double-Edge, while the rest of the EVs are put in Special Defense for maximum special tanking power. If your team is especially worried about Hydreigon, you can invest 20 Special Attack EVs, which allows Clefairy to OHKO Hydreigon with Moonblast. 12 Special Attack EVs also allow Clefairy to 2HKO Terrakion with the moves.
A Sassy nature with 0 Speed IVs is also a good choice for Clefairy, allowing it to take deadly special attacks such as Sylveon's Hyper Voice better and making it fairly fast under Trick Room. If your team uses other forms of speed control such as Tailwind and Icy Wind, a Calm nature may be preferable. If you are more concerned about surviving powerful Steel-type attacks, a Relaxed or Bold nature with an EV spread of 252 HP / 212 Def / 44 SpD allows Clefairy to survive a Life Orb-boosted Iron Head from Bisharp.
Usage Tips
Follow Me is used to protect Clefairy's partners from attacks, and it should be used often to give Clefairy's teammates the chance to set up. If your team encounters Quick Guard or Wide Guard, use Snatch to steal them for your own team, aiding Clefairy's teammates' offensive presence. Snatch is also a great move to use against a predicted Tailwind. Meanwhile, Heal Pulse should be used often to keep Clefairy's bulky teammates on the battlefield longer. If using Helping Hand, targeting spread attack teammates maximizes your team's chance of dealing massive damage. Don't be afraid to let Clefairy go down if it's taking a lot of nasty hits. It's better for Clefairy to do its job for a little while than sit around and not do anything later on.
Positioning is an especially important aspect of Clefairy in Battle Spot Triples. It may be tempting to place Clefairy in the middle to make maximum use of Follow Me, but be warned: getting hit by three attacks will often knock Clefairy out, meaning that your team will lose its Friend Guard support in one turn. To make the best use of Friend Guard and Follow Me, you'll usually want Clefairy on either the left or the right side of your team. Just make sure that the Pokemon in the middle benefits from Clefairy's Helping Hand if you're using that.
If you find that Clefairy's usefulness is going down, you're late in the game, or you just feel that Clefairy has one particularly good shot of shifting the match's momentum to your favor, this is when sending out Clefairy to the middle can be a good choice. This allows Clefairy to use Helping Hand on either partner and target any Pokemon with Snatch. Because this makes Clefairy a big target, using Protect to delay attempts to KO Clefairy is especially useful when in the middle.
Team Options
Pokemon that can set up with Substitute or boosting moves, such as Mega Kangaskhan, Mega Metagross, Mega Salamence, Whimsicott + Terrakion, and Heatran, are all great partners. Tailwind and Trick Room users such as Suicune and Jellicent greatly appreciate Follow Me's redirection support and Friend Guard's defensive boosts. Given that Clefairy has access to Snatch, Pokemon that strongly dislike Wide Guard and Quick Guard such as Sylveon, Mega Charizard Y, Mega Camerupt, Talonflame, and Hitmontop are great teammates. Fire-types, such as Heatran and Mega Charizard Y, can take out opposing Steel-types. Dragon-types such as Hydreigon and Steel-types such as Aegislash and Heatran naturally complement Clefairy. A rule of thumb when using Clefairy is to make sure that its partners are able to make up for Clefairy's extreme lack of offensive presence.
An important aspect of Clefairy's Follow Me support is that it will redirect placement-ignoring moves such as Brave Bird, Dark Pulse, Aura Sphere, and Dragon Pulse. This makes Pokemon such as Hariyama, Aegislash, Mega Metagross, Bisharp, Mega Kangaskhan, Hydreigon, and Mega Salamence great partners, as Follow Me allows them to avoid these moves.
As a special note, Clefairy especially loves Trick Room support, as it gives it the Speed to use Heal Pulse to support teammates, and slow spread attackers such as Sylveon and Mega Camerupt are especially deadly under Trick Room. Some good Trick Room users to consider include Cresselia, Meowstic-M (which can use Safeguard to save Clefairy from Dark Void), Gothitelle, and Jellicent. Another special teammate to consider is Mega Blastoise. Clefairy supports Mega Blastoise with the very valuable Heal Pulse to restore Water Spout's power (the move is especially deadly under Trick Room) and Clefairy can also remove Wide Guard with Snatch, while Mega Blastoise supports Clefairy by slamming Bisharp and Aegislash with its Mega Launcher-boosted Aura Sphere and Dark Pulse, respectively.