In neutral, he's going to be spacing aerials most likely, which is when you want to run up shield (when he lands with an aerial). Also, if you block a spaced fair/bair/nair make sure you don't try to grab; you can try to nair then fade back or fair or dair or something. If he keeps retreating with aerials, you can just control space with back air. Marth can actually dash grab you when you're crouching so crouching is really only great if you're in front of his shield or just landed with a really late aerial that you can bait him into trying to grab, since these are really the only situations where he'll be a doing a standing grab and you can crouch. It's kind of tricky since he has a huge advantage in neutral, but you should do alright if you punish hard enough.
Landing is really hard, but the best advice is to use your jumps. On FD, you can bait Marth into up airing you and then avoiding with a jump, and that will usually give you some time to land. Also, this might sound obvious, but make sure you look at where Marth is and what he's doing lol. A big part of avoiding getting juggled is reacting to what Marth does since he's the one putting pressure on you. On platform stages, make sure of platform wavelands! You can actually make your movement pretty tricky if you are good at moving on and off of platforms quickly with wavelands.
This is pretty general advice and just stuff I have noticed from playing good Marths, but hopefully it helps you out.
