A Devon scope.
go east then south of fortree city til you see a brige
You have to find Steven and he'll give you Devon Scope and you use that on the invisible object.
You first need the Devon scope given by steven at the route east of fortree city after getting the Devon scope you can enter the fortree city gym.
Fortree City. Follow route 118 across from Mauville city and you should reach it. However, a Kelceon is blocking the enterance to the 6th gym. Find steven on the route next to fortree and he'll give you something so you can see it.
continue past fortree to reach a bridge. steven, the champion, will tell you to defeat a kecleon. catch it or defeat it. he will give you an item that will do what you need. click on where you cant walk near the gym. the kecleon will run away
The sixth badge is at the Fortree city gym.(The city full of trees). The gym is blocked by something so first continue your journey east of the the city to find Steven.He'll help you! Then go back to fortree and use the item you got from Steven to clear a path!
From Steven at Fortree city. The thigh blocking you from going into the gym is an invisible Kecleon.
You must get the Devon Scope from Steven on Route 120, then you return and press 'A' when your reach the invisible barrier blocking your way to the gym. (Its actually a Keckleon in stealth)
I believe that Steven gives it to you in Fortree City.
Steven gives it to you on the bridge to the west of Fortree City.
Fourth is at Fortree City, right? You need to walk past the city and you'll find a bridge there you'll meet Steven. He'll give you Devon Scope. Use it to the invisible thing that blocks the way to the gym.