Follow the path (don't forget to use the nearby Rock Climb ledges to find the Life Orb) and you will see two Galactic Grunts. You'll overhear their conversation that Charon has replaced Cyrus as Team Galactic's leader, though it is doubtful if Charon will be as effective. Go through the entrance to the volcano.
Mars' Bronzor has evolved into a Bronzong. It can use Light Screen to halve out the damage caused by Special moves. Her Golbat and Purugly are about the same so the same tactics you used at Spear Pillar can bring them down.
Jupiter will face you next. Her Bronzong knows Reflect to halve out the damage caused by physical attacks. Her Golbat and Skuntank are about the same as well.
After the battle, the two decide to quit the team as it is not much fun without Cyrus. They decide to journey to find the Distortion World. Charon doesn't care and leaves with the two grunts to find the Magma Stone. Using Strength, move the boulders to the right of you carefully and go up the stairs. With the aid of Strength and Rock Smash, go through the large door to the main room.
After you go north for a little bit, Buck will appear. After recognizing that Team Galactic is here, he will decide to team up with you as a tag partner. Like Marley and the other tag partners, he will also heal your team after every battle and Wild battles become double battles.
This room is so large it is recommended to stock up on Max Repels to make traveling through it a little less annoying. It is also highly recommended to take down the Trainers in this room as while you have Buck with you, since he can heal your team and the Trainers' Pokémon are between Level 57 and Level 61. Just remember that some items are inaccessible right now since you can't use Rock Climb as long as Buck is with you.
To make it easier to find the exit, head east past the first set of stairs until you see the path turn north. Keep going straight until you reach a dead end. Go down the nearby stairs and follow the path west, smashing a rock on the way. Continue until you climb some stairs and see two Trainers nearby. Turn left and follow the path to the northwestern part of the room. Go down some nearby stairs and move the Strength boulder. Then, you will be at the exit. Once there, Buck recognizes Team Galactic is inside and heads in.
Inside, Charon is unhappy that Mars and Jupiter quit and won't help him. He plans to use Heatran to control the volcanic eruption to gain millions. Noticing you and Buck, Charon quickly grabs the Magma Stone and gloats. However, a mysterious Trainer sends out a Croagunk to get the stone. This trainer is actually Looker in disguise and, as much as Charon denies it, Looker has him arrested while the two Galactic grunts with Charon run away.
Taking both you and Buck outside, while Buck goes to return the stone, Looker mentions his investigation into Charon's activities. He also states that now that Charon was arrested, Team Galactic is no longer a threat. After Buck returns, Fly to the Survival Area.
Survival Area (second visit)
When you arrive, you will see Buck and Barry right by the house that was locked up earlier. This house is Buck's, and it is also the Battleground, where you can rechallenge the Sinnoh Gym Leaders and all five of your previous Multi Battle partners. There are four Trainers in here each day, and they can only be battled once a day. Talk to Buck and his grandfather, who will tell you the legend about Stark Mountain. This is important— if you don't talk to both, Heatran will not appear when you go back to Stark Mountain. Besides the Gym Leaders, the Elite Four's teams have also been upgraded as well, providing a harder challenge should you revisit the Pokémon League. Barry will also remain outside the Battleground building and on weekends, you can rechallenge him to a battle once a day.
Just remember that after 10 Hall of Fame records, Barry's team will go up by 10 levels and then by 20 levels after 20 Hall of Fame records. The amount of prize money you receive is also increased by 1000, and finally 2000 when Barry's team is upgraded!
Heatran is Level 50 and can be a dangerous attacker. Use a Water- or Fighting-type move to take out a chunk of its HP, then continue hitting with weaker attacks until its HP bar turns red. Then, throw Dusk Balls until you catch it. To avoid the risk of getting defeated by Heatran, heal your team often.
This route can be a bit of a puzzle to navigate. You'll need to use the muddy slopes and logs to access the Berry patch and all of the items. For a shortcut out of this route, from the area where you started, head for the northern area and follow the path with your bike and you'll only have to deal with a few Trainers and breakable rocks.