from my experience, Sheik and Sonic are the biggest issues with minor issues through Fox and Pikachu. Generally, ZSS is probably your best comaIn to use. Pit is also a good choice as well as Pit goes close to even versus Sheik and does decent versus Sonic.
Against Swordfighters, You really just want to play PK Keepaway, respect their range, and find an opening. Most swordfighters lack great Frama Data or most likely all, didn't really look. Versus, Robin, you force Robin to approach unless Robin has a book, but you also don't want him to charge ArcThunder/Thoron. Robin can't really do much besides approach you with an aerial or bait out your PSI Magnet, but Robins approach options are very telegraphed. It also doesn't help that you heal literally nothing besides from Elthunder and Elwind.
Against Marth/Lucina, Play PK Keepaway until they do a laggy move(Smashes, Dair, Nair, missed grab, All of their specials) and punish with Nair, Fair, PK Fire, Zair, or Grab depending on the situation.
Against Ike. I have no experience with Ike, but I know Ike has some laggy attacks, and do the same as you do vs Marthcina
Vs Shulk. You got to play really patient. Shulk is a spacy. What Shulk doesn't like is projectiles and being comboed. Abuse both of those. When he goes into shield, just camp him until Shield goes off because you are not killing him with Lucas's limited Kill Options. Although Down Throw to Footstool is guaranteed at normal kill percents, so if you get a grab, you can kill him earlier than most characters.
Vs Link-You want to play a little bit more aggressive. Do not try to bat Arrows as Link can hold it and hit you over. Respect that grab range and remember you can heal from the bombs.
All i got for now.