I just finished completing my ribbon buddy Ampharos, so I can help you here!
Contests are not hard, the process is just long. There is no cap on how many Pokeblocks you can feed your Pokemon anymore (unlike in R/S/E) so you can literally just level them up until max without having to worry about it being "efficient" or anything.
Anyway just pick a Pokemon that can learn a reasonable number of moves so you can switch them when you do the different contests. I did it with Ampharos who doesn't exactly have a vast movepool, so as long as you don't pick something like Magikarp or Caterpie it should be fine. Then max its contest stats, take it to the Pokemon Fan Club in Slateport and get all 5 coloured scarves from the old man. Then it is as simple as putting the right scarf on, along with a few moves of the right contest type, and just doing them all. Trust me, the contests are extremely easy and it is basically impossible to lose.