Question says it all
The most basic way to do this is to teach a Male Muk Acid Armor at level 43. Then breed it with a Gastrodon so you get a male Shellos with Acid Armor. Then breed with Dragalgae to get a male Skrelp with Acid Armor then breed that with a Goodra so that your Goomy has Acid Armor.
The source doesn't list the Pokemon I listed, but that is how you find Pokemon with those moves, you have to continuously trace it back until you find a Pokemon who learns the move you want via level up.
Hope I helped! :v
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