Instructions
Go to Route 42 (connects Ecruteak City to Mahogany Town).
Use a Pokémon equipped with Surf and Waterfall to reach the middle entrance to Mt. Mortar.
Head to the right and Surf across the lake. Surf across the two lakes to the north and then head left until you hit a wall. Drop down the ledge immediately below you and continue heading left until you hit a lake.
Surf north across the lake until you hit land and then head to the right until you come to another lake. Surf north across this lake and then head up the staircase to grab the dragon scale.