Get ready for a chain breeding.
You gotta breed a Shellos/Gastrodon with and of the Pokemon here or here. The end result they must be male.
After that's done you gotta catch yourself a female Skrelp and breeding it with your newly found Shellos that has Acid Armor. The end result you need a male Skrelp.
Finally, you take a male Skrelp knowing Acid Armor and a female Goomy. Now you are done!
Sources: Bulbapedia page on Goomy and the DB