You have to make pokéblocks from blue berries and feed your Feebas with them. Feed it with pokéblocks that raise Feebas' beauty. For this use Kelpsy berry for example.
My suggestion is to get Gold pokeblocks. Easiest way? Get Leichi berries and mix them with the old man or the groups. I got my Gold pokeblocks that way. The beauty will increase, mine did.
I think that the berries that are more rare than others, are better than the common ones. Try to get the best berries in your bag and make them pokeblocks. Feed it with blue pokeblocks and there you have the Milotic.
