Oh boy. I registered in this forum exactly to ask for advice on SSB3DS (so nice to meet you all).
Let's see... My win ratio is below 15% and my top was around 20%, so I guess I can say I do suck badly at Smash, especially For Glory. I was suggested to play using aerial combat (which I didn't do because I tend to get my controls wrong) and my most recent losses were exactly because of that - I don't time my recovery correctly and I lose the life. Moreover I tend to get overwhelmed by other fighters who tend to act and attack faster (and my character, Lucina, isn't exactly slow). And I'm pretty sure it's not just a bad day, I can't even win against the CPU at high levels (Classic Mode's Master/Crazy Hand is impossible at level 6, and I tried to beat All-Star Mode with Lucina at Normal and I lose).
It's really frustrating to lose this much. I tried to learn how to do short hops so I could do aerial attacks at low heights, and when I manage to pull it off I don't even nibble my opponent. I tried to do aerial combat only to fall off the stage. I tried to learn I shouldn't do smash attacks all the time and... well it did improve my tech, but that's about it.
Help?
Don't be disheartened! With every loss comes a step closer to victory. Don't beat yourself up for losing in Classic/All-star. It's really hard to beat them unless you have equipment.
For fighting people online, I'd argue the three most important things are mind games, edgegaurding, and spacing. You can practice short hop fairs with Lucina all you want, but until you know when to actually use them you won't win. Online I find there to be three kinds of players: (1) those who roll around and spam projectiles, (2) those who roll and approach with only dash attack, and (3) those who are actually good.
For the people who spam projectiles, swat them away with Lucinas fair. If you notice that they charge in with a move after their projectiles hit, a good idea would be to use Lucina's counter to nullify the projectile and their approach. For the people who approach with dash attack, just shield whenever you feel like they're gonna bust it out. Then shield grab them and toss them off stage. If you find them rolling from one end of the stage to another, literally just down smash while they're attempting to escape and you should stop they're scrub tactics.
For people who are actually good, you'll have to implement mind games, spacing, and edgegaurding.
For mind games, let's say you notice your opponent is approaching with aerials, so you up-tilt them to punish their approach. They notice they keep getting shut down in the air by your up-tilt, so now they're going to mix it up. Instead of coming at you with an aerial, they're going to approach you with an air dodge to evade your up-tilt and punish. If you're a step ahead of your opponent, you know that they want to mix up their approach to avoid your up-tilt. So instead of up-tilting you choose to do nothing, they air dodge and hit the ground with a lot of lag, and you can punish however you see fit.
Spacing is extremely important in this game if you want to avoid getting shield grabbed. Basically you want to hit your opponent with the tip of your hitbox. Spacing is dancing outside of your opponent's range fast enough so you can get him in range for your attacks. Memorize the range of your character's moves so you'll know when to use them based on how far your opponent is from you.
For edgegaurding, I'm not sure what options Lucina has as I don't play her, but I'd go to her thread and ask people who know what they're talking about. Edgegaurding is very character dependent, so learn the recovery options of each character.
Sorry if this is a lot of info, but honestly the best advice is just to practice. The more you play, the better you get. Also watch videos of high level players use your character, it'll give you a lot of insight. Hope this helps, man! Good luck on the road for glory.