You need to make as many Dry poffins as you can. The stronger they are, the higher the beauty will rise. Dont get a Pokémon that has an adamant, impish, careful, or jolly nature cuz those natures hate dry poffins. Modest,Quit, Mild and rash natured Pokemon love dry food.Also to get the strongest poffins, get in a group and use you driest berries (micle berries are the driest ,berries).
In Pokémon platinum you can buy a level 32 dry poffin.Keep in mind that you need to have a lot of money for this.
But I have heard that you are asking about white. In white you can't reach max beauty since it doesn't exist in Pokemon white version. If you are trying to evolve a feebas, then I must tell you that the evolution mechanics have changed for feebas. In generation 5, to get a Milotic you must trade a feebas that is holding a prism scale. You can also catch wild milotic on route 1. You must surf and use a super rod in bubbling spots. You must keep in mind that milotic only has 5% of appearing so you must keep trying.
Hope this helped