...I'm really shocked and disgusted at myself for saying this...but Leavanny sucks. It has an O.K. movepool, but its Attack stat (or, more accurately, its drop-dead-Lord-save-me-I'm-gonna-die/horrid,horrid, HORRID Special Attack stat) cut its options in two. Not to metion its somewhat-O.K.-leveling-at-mediocre Speed. Levanny can't play the brave defender, either, due to 1) its mediocre physical Defense, 2) its lousy Special Defense, and 3) its slew of weaknesses out there (especially the ever-commonly carried Fire-Type) turns it into a joke that fellow joke (like Unfezzant and Audino) could sneer at (if Audinos can sneer, or like sneering). But there is a small chance that Leavanny has at redemption: Baton Passing. Here's a possibility:
Whatever nature you see fit
This will help you set up a physical Sweeper.
Substitute to get a free turn for the dude/chick you're Baton Passing to (or for Leavanny if the opponent wakes up early),
Agility to boost Speed for Leavanny and [insert Sweeper's name here],
Swords Dance to boost Attack for Leavanny and You-Know-Who,
and Baton Pass to hand over the stat changes.
Also, you could possibly do Grasswhistle to make 'em fall asleep (lousy accuracy, though) in exchange for Agility, or X-Scissor to deal with a Prankster Whimsicott who just Taunted you (also in exchange for Agility).
This thing dies so quick that I won't bother to list possible pals for Little Miss Deadweight.
If you're carrying Archeops with any of his STAB moves, Levanny dies. If you're carrying Volcarona, Levanny dies. If you're carrying any Fire- or Flying-types in existance, Leavanny instantly dies. Make Leavanny do her thing quickly, or she will instantly be destroyed in a blaze of holy fire. And if the opponent pulls a Whimsicott out of their armpit, and your Leavanny isn't carrying X-Scissor, get the heck outta there.
Overall opinion: Leavanny was made for nursing babies. Her battling abilties prove that. Oh, Nintendo, what the heck were you thinking...
What will Generation VI bring the metagame?
I'm hoping for a large helping of the first option along with a healthy smattering of the third option.