I play with a Corrin frequently so I'll put my thoughts out in a semi-organized fashion:
Fox definitely wins. Our damage output and speed really help in getting away to reset to neutral if we need to and punishing any option that Corrin picks. We don't win by a large margin, but it's less stressful than you imagine once you learn the way Corrin works.
The Neutral:
A lof of the neutral is played just running around outside of pin range trying to bait out an option. One thing to remember about Corrin is her aerials are quick and ac very well. So what you need to do is run in her range then run out really quickly until they commit or you get a read for a grab/dair/whatever. When you see her pin the ground your best bet is to just hold shield. If you shielded the pin Corrin is GOING to take damage. Just keep holding shield and punish what she does on reaction. We're quick enough that we can catch her no matter if she jumps, kicks, or cancels and this can lead to a large punish.
Utilt starts getting utility around 10-15%. Before this she can shield, and after she can nair. So shield every time after utilt just in case. She falls slow enough that she'll nair, we'll shield, and we can utilt again. From there it's pretty easy to juggle her anywhere from 30-50% more if you can read their DI and pp towards where they are.
Landing kill confirms isn't hard unless they're shielding (course). Corrin is going to try and keep you out with sh nair so just jump at them, they'll sh nair, and then Fox can ff nair/dair into usmash. Usual stuff. Just change up your approach depending on the player.
No reason to use Illusion in neutral unless you have a hard read. It's NOT safe in this MU because of the threat of DFS/B and eating 25%+. Unless you're in a tech chase situation, don't even bother with it. Corrin is also fast enough that she can pin you while in the ending lag, soooooo
Reflecting DFS is always worth it if you can, just cancel it with jump and punish. Most of the time an usmash will do the trick, but I want to lab out other follow ups that you can get. There's the potential for uairs, bair... really anything if you can time it right.
The Advantage:
We demolish Corrin here. Her aerial options are good, but aside from fair they're all really laggy. She's also mid-heavy weight so she falls into uairs perfectly. A good option against a Corrin without a DJ is to jump and bait out the counter and punish OR - if they like dair - just walk below them and bait it out. You MUST shield UNTIL Corrin hits the ground and the green "shockwave" comes out or else you'll eat the landing hitbox and get punished. Once this happens grab/utilt or whatever and get back in advantage.
Offstage we destroy Corrin. Fair AND dair can trade with her up B (which is invincible from frames 10-17 and the hitbox comes out on frame 18) and net an early kill. Imo, it's worth going for EVERY time as you'll only eat 2-3% if you get hit and don't trade, but if you do it could potentially be their stock. The risk/reward is heavily in our favor. If the Corrin saved their double jump go for the fair dragdown to footstool, otherwise they'll recover.
The Disadvantage:
This is the hard part of this MU. Corrin's aerials are MADE to combo and kill off the top. Landing vs. a Corrin is very hard since her hitboxes are so large and last a very long time. We are juggle food. Once you hit about 80% air dodging can end up with you losing a stock to uair (which did NOT get nerfed in kill power, btw). Your best bet is to just reset to a platform and hold shield until you see an opening or, if you can Illusion to the other side of the stage safely, do it. She MIGHT be able to pin you if she was too close which can kill you around 110%, so watch out for that.
Offstage is scary. You really need to mixup your recovery lest you eat a DFS/B! I've died sub 70% due to a bad Illusion that got DFS/B'd and it's not fun. Get good with your Firefox angles and try to save your double jump as much as possible. If you keep getting fair'd, it's ok. That won't kill until 130%+ so be thankful it wasn't an uair or bair! Once you manage to get on the stage you should be pretty good to get back into it.
If you end up around 130%, just laser camp until you find an opening for a hard punish. Dthrow or Uthrow will kill you if they get it so just run around and play safe.
There you go! A quick write-up on this MU. It's tough, but certainly in our favor. Both characters damage output is good, but Fox's great neutral and advantage state are hard for Corrin to contest if the Fox is playing a more bait-and-punish style.
