There isn't a single best answer to any problem in Smash, let alone something as massive and dangerous as Marth's Fsmash. My advice to you would be to get a hold of the basics. If things like Marth's jab, one of his lesser used moves, is giving you notable trouble, you need to focus on your fundamentals.
Spacing: control Lucas' movement such that you stay just out of Marth's approach range, observe what moves your opponent uses most when you're in the neutral game (which is basically at the starting positions or something similar where both players are on even ground in terms of stage control/spacing/etc), predict what moves they'll use to approach you in the neutral, and punish with Lucas' insane combos and movement options.
Lucas has a lot of options here, especially when you consider dash dancing (don't just spam left/right, but rather try to do it a little more slowly [you'll need to practice the timing] so your opponent thinks you might approach/gets scared and throws out a move that hopefully whiffs and you can punish; also mix up randomly when you actually approach, what you approach with, and when in your dash dance [options here are shield, dash attack, crouch cancelled moves, grab/jump-cancelled grab, nair, fair, dair, magnet, jump>magnet, jump > djc > magnet > whatever, PK freeze, jump > pk freeze, jump > djc PK freeze, B-reverse magnet into jump cancelled whatever, etc. There's literally way too many options probably lmao]).
Aside from that, work on familiarizing yourself with your character for when you do get in on your opponent. How fast can you do stuff? What combos into what? When should you stop comboing and maybe hesitate/mix up? Think those kinds of thoughts. Familiarize yourself with all the options I didn't list, because there's a ton. Watch videos of top Lucases like Neon, Oracle, Pink Fresh, etc. I figure you'll have your hands full enough with this amount of information alone, so start with that.
And as always, ask questions of anyone willing to answer.
