You reach Undella Town after going through Reversal Mountain with Bianca.
Use your Dowsing Machine and go north to find a Shiny Stone. Go east from the miner, up the stairs, then down south to get the Berry Juice that you can see on the ledge.
Hugh will challenge you when you go up to the Route 13 exit (to the left of the pokémon center) so don't do that if you aren't ready.
Talk to the guy on the left side of the pokémon center to get a Prism Scale. Talk to the sunglasses guy on the right side to unlock the Shocking Shopping Funfest Mission.
Use your Dowsing Machine south of the pokémon center to find a Pearl. Go west from there to find a hidden Heart Scale. Continue west from there to find another hidden Pearl.
You can optionally go east from the Pokémon Center to Undella Bay. Bring a pokémon that knows Surf. You can also optionally explore Route 14 to the south. Just walk south on the Undella Town beach. You can find some items and battle some trainers in those places, but you can't go any farther.
When you are ready, go toward the Route 13 exit, which is to the left of the Undella Town pokémon center. Hugh will appear. He has a level 39 Unfezant, a level 39 elemental monkey (Simipour, Simisage, Simisear) that is weak against your starter, and a level 41 starter (Emboar, Samurott, Serperior) that is strong against your starter.
After you defeat him, you can continue on to Route 13.