Tell me some compatible parents.
Muk learns Acid Armor at level 42 (Grimer at level 40) and is in the Amorphous egg group, breed a male Muk with a female Magcargo/Slugma and BOOM Acid Amror Magcargo.
Breed your Macgarco with a Muk that has Acid Armor.
Source: http://bulbapedia.bulbagarden.net/wiki/Acid_Armor_(move)