Well complete Subspace Emissary in one go and your good they need to go back to the times of Melee where you do something difficult with any character or do something simple with a specific character they need to do that again this time.
Brawl is easy when comes to unlocking Anything. Melee is fucking Impossible Game when it comes to unlocking Shit. I Hope SSBU is in-between like when You unlock the 15th or 20th Character it'll start getting harder (if they have that many characters in Nintendo that is)