According to Psypoke, to get a Milotic in “Pokémon Emerald,” you have to find a Feebas, increase its beauty rating to at least 170 and then have the Pokémon grow a level. There is no indication in the game of a Pokémon's beauty rating, which makes is impossible to know for sure when Feebas will evolve into Milotic.
Much in the same way that evolving Feebas is difficult, trying to find a Feebas in the wild is also a challenge. This is because Feebas' location in “Pokémon Emerald” is randomly chosen at the start of the game. This Pokémon is only attainable by fishing from certain squares along Route 119. There are 400 different squares from which a player is able to fish on Route 119, and Feebas only appears from six randomly selected squares.
The easiest way to increase a Pokémon's beauty rating is to give it Indigo or Blue PokéBlocks. Unfortunately, not all Pokémon like Indigo or Blue PokéBlocks. The color PokéBlock that the Pokémon likes is determined by its nature. If the Feebas caught is modest, quiet, mild or rash in nature, it likes Indigo and Blue PokéBlocks. If the Feebas has a nature that is careful, adamant or impish, the player needs to catch a new Feebas with a different nature. This is because a Feebas with those natures hates both Indigo and Blue PokéBlocks.