It depends what mon. I mean, i want a salazzle with fake out. That would be boss, but you have to breed a poke with fake out with a poke that can learn fakeout through an egg move, and then be able to breed that poke with a salandit to get fake out and then probably breed that salandit again until you get a female that by chance gets that move from a parent poke, note its still only a chance. You will probably actually have to breed 200 salandits plus to even get a fake out salandit thats female. Also natures are super important, so getting the perfect egg salazzle thats female is like 1/1000, but thats the worst i can think of.
Basically breeding pokes for special moves or special abilities is the way to go. I spent a long time using berries and power items to get perfect Stats on my pokes. I never actually bothered with breeding. Some rare berries lower stats so you can use the power items bought to increase more desired stats.
There's a feature you can learn from someone standing at the battle tree that shows pokemon potential which can help determine if your poke is boss or not.
Note though, one of my pherimosas registered as the highest grade poke, but it still paled in comparison to my pherimosa with -def/+speed that almost had a level lower grade of potential.
ps, you're going to want to plant a lot of those rare stat lowering berries in poke pelago. They are super rare, i think u can get them at one tree in the whole game.