Marth vs spacies:
At 0% it's a much different game than when they're at that lovely 30-60%. At very low% you want to rack up damage as safely as possible, which is done by hitting them with a tipper fair, tipper dtilt, or best of all, grab (cg). Marth's (DD) grab game is still untapped. You can watch PPMD for a virtually perfect DD game in neutral, but one day there will be someone techchasing like wizzy's falcon/m2k's sheik with marth, I know it. When juggling it's good to know the importance of the difference between a tipper uair and a non-tipper uair so you can properly follow up. Non-tipper uair can lead to regrabs at fairly high%. Around 55% to 70% you're looking for uthrow to fsmash (above 60% requires a baby step forward/back if they DI fully). Try to tipper it, but anything that sends them offstage the earliest is best. Edgeguarding them is all dtilt and jab basically. Know when to double jab, for low%/SDI/shortened side-b. Marth has the best edgeguarding tools in the game, so resist temptation to continue combos OR give up stage control; always try to get them offstage. When they're at high%, keep calm and don't get greedy or antsy for the ko. Keep your neutral game solid and take the ko when it comes or you'll get bodied. Shield DI and ASDI/cc his approaches for easier grabs and more oos options. Reverse up-b oos is wicked when you throw off their approach (like, force them to hit your shield early/high up and mid the L-cancel... perks of having a great neutral game) and is underused. Something to keep in mind. Also out of fthrow, wd forward ftilt can often be better then fsmash (not great for spacies but still). Shield DI shines and grab/fair oos. PS to reverse up-b. ASDI grab (down and away or down and in) jabs and other low kb attacks. Safely put damage on him, get him offstage. PS falco lasers. If you are hit with one midair fair instantly. At the very minimum wd oos or SDI the laser.
^^ horribly organized thoughts, but good stuff, I promise