Zelda vs Falco is easily in Falcos favor. There are so many things that Zelda has to watch out for and the margin for error isn't high for her.
At low percents Zelda will get walking chain grabbed. If caught at 0 Falco can take her to about 40 or so and if he spikes her he can get a mix up. Taking the off the stage spike doesn't help much because you're forced to up b more or less right away or risk getting spiked by Falco again during the up b start up animation. And even if you Up B on stage safely Falco can and will follow the up b and continue hitting you. My best advice is to learn to tech the spike and smash DI it into the stage and try to make use of Zelda's long tech roll animation. That is your best bet of escaping the mix up. It's not a sure escape but it's better than nothing. Also I would still be ready to jump out at anytime just in case the Falco makes a rare mistake.
Din's is mainly used to annoy. It's slow and easily avoided by Falco. I'm pretty sure he can clank with it in the air with some of his moves as well. Possibly dair, bair and uair. swordsaint is correct when he says that most Zeldas will auto pilot dins against off stage opponents so Falco can use that to his advantage in certain instances and use the Dins for a vertical boost or to recover past it and punish.
Falco can edgeguard Zelda on the other hand quite easily and with little risk to himself. Bair, shine, and f tilt are great tools for either keeping her off stage, eating her double jump or knocking her out of her up b start up. Lazy Zelda recoveries can and should be punished with bair or spikes. Against Falco I highly suggest mixing up your recovery. One read or careless moment and you can kiss your stock good bye. Do not try to challenge with falling nair or dair unless you absolutely need a gimp or something. It's not worth the risk and a well timed Falco bair or uair will beat it most of the time.
Zelda can rack up damage at low percent with upsmash but after the first few games the Falco will most likely realize (if they didn't know before) that they can just jump away after the first up smash. There may be rare cases where Falco gets caught with the ending part of the up smash which would make a second one possible, but over all Falco should be able to escape F and U smash no problem. The silver lining is that Zelda can get kills early with fresh sweet spots and up airs/tilts. It's not something that can be fished for, but it can be an equalizer in a pinch. So long as Zelda can keep up in the damage, she can also try to use up smash as a revenge kill move since at about 110-115 it should kill Falco outright when fresh. I don't like to rely on up smash personally, since if I miss I know I'm going to eat a big punish. I also recommend saving down smash for kills (120+ just to be safe) because it can put Falco in a position where he might end up below parallel with the stage depending how he DIs it.
On the ground Falco's jab shuts down pretty much anything that Zelda can try to do. It snuffs f-smash, which is what most Zelda's will try to spam/poke with. Down tilt can stop his illusion but most of the time Zelda's will not space it right and the invincible 1/3 of the illusion will just go right through it. Unless the dtilt trips, it won't lead to much because he can SDI it. That and most Zelda's spam it at any percent and waste it. Zelda d tilting an opponent at 0 just gives her frame disadvantage. I would try to wait until 50% or so before going for it.
This match up is such a pain because Falco can take a more reactive approach while Zelda has to be proactive. Zelda has to take the initiative and keep the damage output either as even as possible or in her favor. Once she falls behind the fight just gets harder and harder as Falco will rack up damage faster and can erase stocks with edgeguards and punishes. The Zelda has to be good at reading and take some chances at times to win this fight. The Zelda should pay attention to any patterns in recovery, spot dodge usage and or illusion usage. Punishing at the proper time with the proper move is also important. Zelda has to know when to punish with up air, or up smash etc but only when it'll maximize the usage of the move to make the punish count.
Zelda has to stay patient and pick her spots wisely and she can sorta kinda keep up. I also advise Falcos to watch how Zelda players tend to play on the second stock. Most new/less skilled players will tend to rush the 2nd stock and you'll most likely get a free chance to chain grab. They're over anxious to get a revenge kill and will probably not be spacing as well as they normally would be.
This match is AT LEAST 60:40 in Falco's favor. I say 65:35. 70:30 at worst but I do think that Zelda's kill power keeps it a little closer than that. Falco has the clear advantage, but Zelda can make this doable if they know what to look for and how to execute their punishes.
