Instructions
Travel to Mahognay Town and head north to enter Route 43. Head north through Route 43 until you enter the Lake of Rage. There is a Red Gyarados roaming around the middle of the Lake of Rage that you must fight.
Use a Pokemon who knows Surf and enter the Lake of Rage. Surf out to the Red Gyarados and battle it. Capture or defeat the Red Gyarados to obtain the Red Scale, which you'll need to get Experience-Share.
Head to Cherrygrove City, which is located to the west of New Bark Town and to the southeast of Violet City. Once you are in Cherrygrove City, head north to enter Route 30.
Travel northeast along Route 30 and enter the house of Mr. Pokemon. Talk with Mr. Pokemon; he will offer to trade you Experience-Share for your Red Scale. Mr. Pokemon's house is all-brown and is located in the far northeast corner of Route 30, near the entrance to Route 31.