I would say work on whatever you end up having issues with the most. Try developing clever routines with the CPU to try applying it in real time.
A very simple thing I think which requires routine practice is simply maintaining your ability to control the character at all. Go mess around with the CPU at various levels from say lv1 to lv9. Play with them and focus on a few simple principles.
- You do not want to get hit.
- You want to hit with the tip.
- You do not want to miss.
You do not want to go in randomly doing whatever, but making sure you are focusing on improving some singular or few point aspects at a time.
Just remember not to learn too much from fighting CPUs, especially lvl9's. They have, in a few different circumstances (multiple regarding grabs and power shields) 1-frame reaction times. Also, they have really inhuman DI and tend to teach you bad habits. For example, I 'learned' from lvl9 CPUs that it's a good idea to do a lot of fsmashes and counters, when in reality, it's quite the opposite.
Are there any specific things that Marth mains who don't live in a very good region do to practice? I realize shadowboxing is a viable way to practice however I just don't really know how to do that with Marth when it comes to neutral situations. Any specific things you guys do?
If you have another controller, use it for specifically practicing one or two techniques until you have it down. For example, I'll go into an infinite time match (time match set to where 0 mins would be if you didn't know) with my controller as Marth and my brothers' as Sheik (because she's a pain and learning how to space against her specifically is useful for me). Normally what I'll do is set the stage to FD and just go crazy practicing. What I usually practice is
-Testing out new combos (though it's not too good because a dead controller can't DI)
-Spacing (lots and lots and lots of spacing)
-Wavedashing (including OoS, for the sake of spacing, and even to keep up a PPMD-esque dtilt combo)
-Edge techniques of all sorts (lots of work can always be done here)
-Chain grabbing (M2K-esque)
-Teching/Techrolling (usually have the Sheik controller grab -> dthrow me and I'll quickly switch so I can practice my techs)
Note with the chain grabbing that it's a good practice after a certain point to dash dance -> JC grab
That's about all that I do when I'm practicing on my own. One thing I stand by, though, is that using just about any of these techniques in practice is
way different from using them in the heat of the fray, when you have a lot of other factors to pay attention to.