You are probably referring to the Fight, Resort, and Survival Area. It is accessible in Snowpoint City. Talk to a sailor after toy beat the Elite 4 and he will offer you to go to the Fight Area. Once you arrive, heal your Pokemon. If they are fully healed, go straight towards the entrance of the Battle Frontier. At the gates, there will be your rival, (his name), with Sunyshore City Gym Leader, Volkner, and Elite 4, Flint. At that point you will have a tag battle against the two. After the battle, head into the Battler Frontier and challenge some of the facilities. You must have a National Dex before advancing. Surf through the body of water where you will encounter trainers and grass. Keep surfing until you reach land which would be the Resore Area. Head north to a route in a sandstorm. You can also find the Ultimate move tutor there that will teach any friendly fully evolved starter one of the three elemental Hyper Beams (Frenzy Plant, Blast Burn, Hydro Cannon) respective to their type. Head through the gate and head north into Stark Mountain. You will find a trainer named Buck who will travel with you through Stark Mountain. He will retrieve an item in a chamber and return to the Survival Area. Get to the Survival Area by surfing east of the gate to the sandstorm route. Talk to him a in a house to the left of the Pokemon Center. If you wish, return to the chamber in Stark Mountain to battle/catch Heatran. Otherwise head south to a route that will return you to the Fight Area. Hope I helped.