Instructions
Go to Floaroma Town in the west of the Sinnoh region, and go north out of town to the Flower Field.
Go up to the first person you encounter in the Flower Field; he will sell you a jar of Sweet Honey for 100 Pokedollars.
Find a brown tree. There are brown trees on most of the routes in the game, including two on each of Routes 205, 210 and 212, as well as others on Routes 206, 207, 208, 209, 211, 213, 214, 215, 218, 221 and 222. There are also brown trees in Eterna Forest, Floaroma Meadow, Fuego Ironworks and Valley Windworks.
Go up to a brown tree and select the Sweet Honey from your items bag. Press "A" to slather the honey on the tree.
Come back 12 hours later in real time, go up to the tree and press "A" again. You will encounter a wild Pokemon, one of Aipom, Burmy, Combee, Cherubi, Munchlax or Heracross.
Repeat the process using as many different trees as possible to increase your chances of finding a Heracross.
Wear down the hit points of the Heracross using a Flying-, Fire- or Psychic-type Pokemon -- it is particularly weak against these types. Once its health is very low, catch it using a Pokeball.