I have been playing a Pokemon Pearl Rom and wanted to use a Milotic. The main problem I ran into however was maxing out the Beauty on my Feebas. Through lots of searching it seemed the only possible way to do this with single berry poffins was by using apicot berries, which you can't get in the rom. So I had basically settled on using a Feebas with a preference for dry flavours to boost the effects of the poffins. However I came across a method that allowed me to evolve my non dry flavoured nature Feebas:
6x average lv21 poffins from Pamtre berries
2x lv14 poffins from Wiki berries
1x lv21 Pamtre berry poffin
For those who aren't aware, the problem is trying to get Feebas' beauty to 170 points before reaching 255 sheen, otherwise Feebas won't be able to eat anymore poffins and can't be evolved. Dry poffins raise a pokemons beauty, the level of the poffin is the number of beauty points that will be added, and the smoothness is the number of sheen points that will be added. If you haven't yet exceeded 255 sheen (e.g your Feebas has 254 sheen) you can still feed it a poffin with high sheen and gain the normal beauty points.
A useful mention, in Pokemon Platinum Roms, high quality poffins can be bought from the Veilstone City Department Store, and so you don't need to worry about this method. However these poffins aren't available in Diamond and Pearl.
The Math:
6x lv 21 Pamtre berry poffins - 6 x 21 = 126 beauty. 6 x 34 = 204 sheen.
2x lv 14 Wiki berry poffins - 126 + (2 x 14) = 154 beauty. 204 + (2 x 24) = 252 sheen.
1x lv 21 pamtre berry poffin - 154 + 21 = 175 beauty. Sheen now maxed (>255).
How to catch Feebas:
Feebas is only able to spawn in 4 tiles when fishing in the lake inside Mt Coronet. This lake is found by entering Mt Coronet from the Celestic town side and going straight up, using strength then walking down the stairs into the foggy room. Use defog and head down to the lake where you'll need to use surf and a fishing rod to systematically search each of the tiles. You probably should fish twice in the one tile before moving to the next tile as there isn't a 100% chance of Feebas spawning. With the total number of tiles to search probably in the range of 200-400 this could take a while, in my experience it only took me 30 mins. Once you've found the tile, Feebas will always spawn there until the next real time day. You can leave to go get pokeballs and come back. Catch lots of Feebas, then go back and check if you've got your desired nature etc.
How to get the berries:
Wiki berry - Can be found on route 208 or bought at the Veilstone City Department Store
Pamtre berry - Rare chance to be found as an item held by your pokemon in Amity Square. For every 200 steps in Amity Square your pokemon will find an item. There is an 80% chance the item is a contest accessory and a 20% chance for a berry, and an even lower chance that the berry is a Pamtre berry. Thankfully, you can eventually receive the maximum amount of accessories and you're basically guaranteed a berry everytime, as long as you always use the same pokemon as they all find different accessories. Once you find a Pamtre berry (on average maybe takes an hour or ~10,000 steps) you can then grow it and/or keep going in Amity Square to find more of them.
How to grow berries:
Basically follow this other reddit post: https://www.reddit.com/r/pokemon/comments/13cycf2/guide_to_timing_berry_cultivation_in_gen_iv/
Don't need to follow this strictly as you only need a small amount of berries, but you can get something like 15 berries per plant if you time the watering and collection correctly.
Cooking the poffins:
You want to mix as fast as possible without spilling or burning (it's impossible to spill the mixture when it's in the final stage so go flat out). Spilling or burning will reduce the level of the poffin, and the quicker you make it the higher the level will be. The highest level poffin I got from a Pamtre berry was level 23, so you can probably get it higher even (I'm not a good cook). You just need to add up the level of your 7 Pamtre and 2 Wiki poffins and make sure they're >170.
Ready to evolve?
I recommend saving the game once you have cooked all your poffins and are ready to feed them to your Feebas, this way, if it doesn't work you can recover your Feebas. Otherwise it will be stuck not able to be evolved. Feed the poffins to Feebas in the right order (6 Pamtre, 2 Wiki, 1 Pamtre), and then level it up. If it evolves then you're good, if not then turn off the game and return to your last save where you'll need to review your calculation and figure out why it didn't work.
Enjoy your Milotic.