For online you have to learn how to play AGAINST specific characters.
Also play patient. It's more important to observe the enemy and adapting to it's playstyle instead of repeating the same things, until they somehow work.
-> Keep your eyes on the enemy(and on luma if he plays Rosalina...), not on your character (you need to be able to still control you character).
-> Reading you oponent's moves is good, but don't make the mistake to depend on it.
-> Instead try reacting fast to his actions. (For example ledge game)
Don't be scared to use your shield against projectile spamming characters. It got buffed in the past and will hold long enough.
If you really are desperate to come elite, you can play on stupid rules and abuse them with specific characters.
But that does not make you a better player.
If your oponent plays Pacman, there is nothing you can do. Pacman players are just better then everyone else, no reason to feel shame.