To basically increase a Pokemon's base stats, go to one of the bridges (for example, Driftveil Drawbridge or Marvelous Bridge). There will be flying Pokemon shadows that appear on them and wings that increase a Pokemon's base stats will appear about 60% of the time; the other 40% of the time wild Ducklett and Swanna will appear. The wings that appear are:

Health Wing - HP
Muscle Wing - Assault
Resist Wing - Defense
Genius Wing - Special Assault
Smart Wing - Special Defense
Swift Wing - Speed

Note: All wings except the Wing will raise the base stats until they reach 255.