Sorry, was busy nomin on pizza. Um, you need to clear out the South Aventurine Forest basically. Remember that defeating Tauros will let the Bouffalant charge in the direction they were facing, pushing logs blocking the path, and defeating Bouffalant will allow you to ride Tauros, which will allow you to jump up ledges.
After, you clear out the logs, you'll notice webs and logs blocking your path. You'll also notice the odd looking trees (hopefully you don't miss them). If you go up to the tree with honey, you'll be able to attract wild pokemon, and in this case, Wild Heracross/Pinsir. You can't catch them since they're too busy engaged in battle, but defeating one of them will allow you to explore further. Defeating Heracross will allow Pinsir to crush logs, and defeating Pinsir will allow Heracross to slice the webs. I can't really tell you exactly where the final Heracross/Pinsir are, but try to start from the most west entrance of the South Aventurine Woods and work your way to the right.
Anyways, that should help, post here if you need further assistance from me/others.