Ledian has all the right moves he needs to be a great Baton Passer or even an adequate lead. Encore is the selling point of this set, and allows Ledian to separate himself from his fellow Bug-type Baton Passers. The main idea when using Ledian is to switch into an attack that poses little to no threat, Encore it, use either Agility or Swords Dance as the opponent switches out, and Baton Pass out to the appropriate teammate. Unfortunately, Ledian's lack of an attacking move on this set means he is utterly dismantled by Taunt; however, Ledian's Attack stat is pitiful and he wouldn't be damaging much anyway. When considering Ledian for a slot on your team, it is important to realize that he is only improving your team of five with Agility and Swords Dance boosts--like an accessory--and that your team should be able to function without Ledian.
The EVs maximize Ledian's surprisingly good special bulk while giving him max Speed to ensure he outspeeds the omnipresent base 80s that riddle the UU metagame. When using Ledian, the best attacks for him to switch in on are non-damaging moves or Grass-, Fighting-, and Ground-type attacks. Ledian should then proceed to Encore the move, use Agility or Swords Dance as the opposing Pokemon switches out, and Baton Pass to the appropriate teammate when the opponent's switch-in is revealed. All of this switching can quickly deplete Ledian's health, especially if Stealth Rock is in play, so Roost can be used in place of Swords Dance to increase Ledian's longevity.
If considering Ledian for the lead position, it is important to remember that he will be forced out immediately by half of the most commonly used leads in the metagame. Examples of leads that immediately force out Ledian are: Ambipom, Alakazam, Moltres, Arcanine, Snover, and any Pokemon with Taunt. Losing against all of these leads may seem overwhelming, but Ledian is perfectly capable of setting up on the half not mentioned. Against any other lead, the best idea is to Agility on turn one. This way, Ledian is able to outspeed the other lead and Encore whatever they used on the first turn. After Ledian has successfully Encored their first move, he is free to use Swords Dance as they switch out. After they have revealed their switch-in, Ledian can then Baton Pass to the appropriate teammate. The item choice in the lead position can vary. Lum Berry is the most useful as it protects Ledian from one turn of status, allowing him to pass at least the Agility he used on the first turn to the appropriate teammate. Focus Sash can be used to protect Ledian from Stone Edge or Fire Blast from leads such as Pinsir, Moltres, or Nidoking, but these leads are rare, so most times Lum Berry will prove more useful.
Because Ledian will be passing Agility and Swords Dance boosts, the best teammates are those that can capitalize on said boosts and cover Ledian's weaknesses. One of the best partners for Ledian is Rhyperior. Rhyperior can easily switch into the Fire-, Rock-, Electric-, and Flying-type attacks that Ledian will encounter, is a great immediate switch-in to Ambipom if using Ledian in the lead position, and really enjoys the boosts Ledian provides. In turn, Ledian can easily switch into the Ground-, Fighting-, Water-, and Grass-type attacks that plague Rhyperior. They both share an Ice-type weakness, but it is an uncommon attacking type in the UU metagame. Other good teammates include those that cannot be phazed out, such as Octillery or Cradily. If using Ledian in the lead position, Pokemon that can switch in immediately into Alakazam or Mismagius, such as Spiritomb or Absol, make good teammates as well. Thanks to Ledian's ability, Early Bird, and the Lum Berry, he can also Agility on the first turn against sleep-inducing leads such as Venomoth or Venusaur, Swords Dance and absorb the sleep the next turn, and have an increased chance of awakening immediately the next turn to Baton Pass to a teammate as the opponent switches out. This strategy is risky, but the payoff is usually greater than the risk involved.