Inside is a Black Belt who demands a challenge before you can pass.
Trainer Battle: Black Belt
Head up the next set of stairs. There is a Doctor here that you can beat to create a healing point.
Trainer Battle: Doctor
Head past the Doctor to the left, around, then stop before the stairs leading up. If you have Strength, you may as well push the boulder into the hole to create another shortcut back. When you’re ready, head up the stairs.
Upstairs, heading south will take you outside, where you can drop down and enter the nearby cave for an item. After picking up this item, you can hop down the ledge immediately to the left to return to the Doctor and backtrack to the point before you dropped down.
Anyway, let’s assume you’re at the point where you just took the stairs going up in the Doctor’s room. Heading north–and not south–will lead you to a flight of stairs. Up these stairs, there will be a trainer nearby who you can either fight or avoid.
Trainer Battle: Ace Trainer
Take the right hand exit. There are two slide points here, take the right hand one and enter the cave you arrive at. Follow the stairs up, the Veteran here can be fought or avoided again.
Trainer Battle: Veteran
Take the stairs you find and then head left, down the next flight of stairs. Downstairs, use the power of Strength to shove the boulder into the hole, creating yet another nifty shortcut. With the shortcut in place, go down a bit, then left.
If you encountered the Legendary Pokemon Cobalion at Mistralton Cave, there will be an entrance here to the Trial Chamber–inside, you will find another Legendary: Terrakion .
Legendary Pokemon: Terrakion
Catching Terrakion is not dissimilar to catching Cobalion or Virizion. But now that you’re, hopefully, at a high Level, you should have an easier time standing your guard.
Terrakion and its fellow Legendaries would make great additions to your Pokedex and Elite 4-beating team, so check them out if you wish.
Return upstairs and exit to the south. Now run up the road towards the final confrontation…