To start, head back to the Pokemon centre in Humilau City (where you beat the last gym leader, Marlon)
1) Head south to the water and begin surfing. It's pretty straight forward here. Once you reach the end, you should find a cave entrance (entering from the top). A bluish person is standing near the entrance.
2) Head south and go down the small ledge, where you will battle a trainer with a red hat.
3) Afterwards, there will be 2 ledges; take the one on the right. It will lead you to stairs in which you will take.
4) When you reach the water, use surf all the way to the right. Then, go up the small river.
5) When you reach the end, you will see a staircase going down (on the left), and some stairs going up (on the right). This is where you will battle the crustle you were wondering about (looks like the big rock on the back of it) - you will need to use the Colress Device on it. (It's standing next to the entrance).
6) Afterwards, exit through the cave in which you will be taken to Route 21.
*It's pretty straightforward from here. If you are confused, watch this video (6:17 - The End)
Hope I helped! :)