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I get how the set is run in theory and on showdown, but according to serebii, riolu can't learn Copycat until far after level 5. So how can he be used in LC?
If both of the parents know a move that the baby can learn via leveling up the Pokémon in the egg will inherit that move. Riolu learns Copycat at level 19 then you evolve it into Lucario (since Riolu is a baby mon it can't breed) and then you breed it with a Lucario that passed through a similar process it'll result in an egg containing a Riolu that knows Copycat at level 1.
If both of the parents know a move that the baby can learn via leveling up the Pokémon in the egg will inherit that move. Riolu learns Copycat at level 19 then you evolve it into Lucario (since Riolu is a baby mon it can't breed) and then you breed it with a Lucario that passed through a similar process it'll result in an egg containing a Riolu that knows Copycat at level 1.