Sweeping Leaf
Leafeon@Life Orb/Leftovers
Adamant nature
252 Atk/200 Spd/56 HP
~Swords Dance
~Leaf Blade
~Aerial Ace/Bite/Baton Pass

Really, the only viable Leafeon set. Between Swords Dance and Chlorophyll, Leafeon is capable of outspeeding and killing so friggin' much. Leaf Blade is the ideal STAB move here, but this is where the fantasticness ends. The rest of Leafeon's at least semi-viable physical movepool consists of X-Scissor, Aerial Ace, and Bite. Ugh. X-Scissor you'll want at least. The last slot, you can either use one of Leafeon's less than ideal moves for a bit of extra coverage, or slap Baton Pass in there to pass on them Swords Dance boosts.

Item is up to you. If you want Leafeon to pack more of a punch, go with Life Orb. If you prefer allowing it to recover HP it loses on the setup turn, then Leftovers are for you. And nobody in their right mind runs Leaf Guard on Leafeon. Maybe if it worked like Hydration it would be better, but...

Ninetales, for a start. With Chlorophyll, Leafeon's gonna want constant sun going. It's also best to partner it with a Magneton or Magnezone, as they can handle those pesky Steel-types that otherwise wall Leafeon to hell and back.

Victreebel is 4x resistant to Leafeon's Leaf Blade and at worst takes neutral damage from an X-Scissor, and in the sun, that Weather Ball is gonna KO. Steel-types in general resist everything Leafeon can throw at them, with the possible exception of *snort* Rock Smash. Also, as Leafeon is tailor-made for sun teams, Fire-types are obviously a major problem, as their already super-effective moves are even more powerful against Leafeon. Darmanitan in particular comes to mind. Nothing a Leafeon is gonna do will take down a friggin' Darmanitan, and Leafeon's likely to be on the recieving end of a STAB'd, Sheer Force boosted, sun-boosted, super-effective Flare Blitz off of one of the highest Attack stats in the game, which may or may not have a Life Orb boost thrown in there! Ouch to the extreme. Weezing resists Leafeon's best moves, can cripple it with Will-O-Wisp, and KO it with either a sun-boosted Flamethrower or a STAB boosted Sludge Bomb, both being super-effective.