Four random tiles in the water of the basement in Mt. Coronet. Just keep surfing around and use your best fishing rod until you find it. Its location changes every day. Give feebas the Max beauty and level up. Here is what milotic looks like(It is at the bottom). The stats are here too when it is level 100. Milotic's Stats Stats at
Level 100 0 EVs 252+ EVs Base Rank Min Max Min Max HP
300 331 363 394 95 62nd Attack
125 156 188 219 60 329th Defense
163 194 226 257 79 176th Sp. Atk
205 236 268 299 100 66th Sp. Def
255 286 318 349 125 18th Speed
167 198 230 261 81 155th Total
540