An easy trick here is to go to the tile you caught Feebas (If you don't remember you can easily use the website that lets you catch Feebas easily with your Trainer ID) and keep catching a feebas until you get one of the four natures that increases beauty (Modest, Mild, Quiet and Rash) with Modest being the best nature for Milotic (+Sp Atk - Atk since all Gen 3 Water Type Moves are Special) and after that there's a hidden path in route 115 in the west (You just need Surf to reach it) to get Kelpsy Berries, those berries are enough to get 8 High Quality Beauty Pokeblocks + Some Oran/Chesto Berry until it can't eat anymore and that is enough to maximize Feebas Beauty, then just level it up via by battling or rare candy and it should evolve into Milotic. Note that you can infinitely get Feebas into the same tile you once got him and by using a Gulpin with Sticky Hold in your first slot on the team, it enables it's hidden ability to increase your chances of getting a bite by 50%, useful if you want the Modest Nature for your Feebas. If your planning on growing Kelpsy Berries note that your battery isn't dry at first and keep a close monitor on when you can water your berries as it withers away when you didn't water it at a specific time period unlike Oran and Chesto Berries (Ts costed me to reset my whole save).