Ledian

Early Bird
Reduces sleep time by 50%.
Swarm
Boosts power of Bug moves 50% when at 1/3 HP or less.
Type Tier
Bug / Flying UU
Statistics
Min- Min Max Max+
HP
55
- 251 314 -
Atk
35
95 106 169 185
Def
50
122 136 199 218
SpA
55
131 146 209 229
SpD
110
230 256 319 350
Spe
85
185 206 269 295
Name Item Nature

Baton Passer

Focus Sash / Leftovers Calm
Moveset EVs
~ Substitute / Swords Dance / Agility
~ Baton Pass
~ Bug Buzz / Silver Wind
~ Light Screen
248 HP / 8 SpD / 252 Spe

This is probably the best use for Ledian's talents. Focus Sash is likely to work well if you use this in UU, particularly with Agility.

Name Item Nature

Support

Leftovers Calm
Moveset EVs
~ Light Screen
~ Bug Buzz
~ Knock Off
~ U-turn / Roost
248 HP / 8 SpD / 252 Spe

Ledian has two good stats, so it may as well take advantage of them. The idea here is to switch into some special attacker and use Light Screen. Knock Off also helps. Bug Buzz is the best way to get something resembling damage out of Ledian. The last spot is a toss-up; if you actually think Ledian can survive anything, use Roost. Otherwise, U-Turn might prove handy.

Name Item Nature

Swords Dancer

Leftovers / Focus Sash Adamant
Moveset EVs
~ Swords Dance
~ Focus Punch
~ Aerial Ace
~ Baton Pass / Ice Punch / ThunderPunch
252 Atk / 4 Def / 252 Spe

Whatever.

Name Item Nature

Choice Specs

Choice Specs Modest
Moveset EVs
~ Bug Buzz
~ Focus Blast
~ Giga Drain
~ Hidden Power Flying
4 Def / 252 SpA / 252 Spe

Ledian does have some good resistances and that nice Special Defense, so, uh, this could work. Maybe. With Magneton/Dugtrio support when the walls and fast Pokémon are dead. Yeah.

Other Options

Double Team, if it's allowed.

EVs

Generally, if you're using Ledian, complement its decent Special Defense stat by pumping HP and using a Calm nature; however, do not max HP as that would give Ledian an even HP stat, causing it to faint on the second switch-in to Stealth Rock. An odd HP ensures it survives with 1 HP if it doesn't take any other damage. For the two attacking sets, use their respective Attack-boosting natures, but don't forget to avoid an even HP.

Opinion

If you're actually planning to use Ledian, then you're probably not looking to use a good Pokémon. Props to the brave people who use it, though.

Counters

"What doesn't counter Ledian?" would be a better question. And the answer is—Luvdisc, maybe.