Rosalina - Everything this character does is stupid.
Both Links - Keepaway sucks for DDD
Samus - Samus is actually a really strong character when you aren't fast and don't beat her projectile game
ZSS - Neutral B is very hard to play against as DDD
Greninja - Can't do much against shuriken spam and reacting to the approaches shuriken spam forces
Captain Falcon - He's so heavy, I find it really hard to KO him before he KO's you in a battle of attrition
Rosalina - Kill the Luma then go for Rosalina, use Ftilt against Rosalina because its going to save your life from these dodges and that stupid dash attack. I've won more against rosalina's this way
Link - Don't rush towards him, you can even use him to practice your shield timing from the projectile spaming, He'll start to come towards you and that's when you start wiping the floor with link.
Samus - Samus (now) is like the D3 reaper because she's just as versatile as D3 in the air, just like link let it spam the projectiles until he starts approaching you, be patient and try to limit your air attacks with Nair Fair/Bair.
Greninja - I've never had any problems against Greninja players, short hop between these spams take advantage of the 5 jump ability you got and pressure him with gordo's and fast falling Nair's.
Captain Falcon - Captain Falcons are not that hard, you can surprise them with close ranged gordo's and inhale ( try to use inhale more often in battles very useful) try to bring him out of the stage.
That's just a few suggestions to add into your playstyle, at best I'd like to see how you play DDD so I can get an idea. See ya