Trying my best to train up a Marth, so I figured while I was browsing this bored, I'd comment in an area of I've got some xp in. heh
I lose to random F-smashing way more often than I should, but I've started to learn a thing or two. A lot of the issues I see with people actually getting hit by this just comes from people just trying to do too much. Wavedashing in, throwing out moves, dashdancing at iffy spots... It all just kind of of puts you in a position to get randomed out.
One of the biggest issues I have personally is getting caught while jumping out of shield. A lot of the super low level Marth's don't feel confident enough in the potential of grabs and so they just toss out an Fsmash while you're in shield cuz they don't know what to do and they know that thing hits hard. Throws me off a ton, cuz a raw smash on shield isn't something you come to expect once you get to a certain level of play.
Best thing I've come to learn aside from the obvious baits and punish is to just stay grounded. Just toss out some solid d-tilt pressure and make sure that if you're in range of that F-smash that you're both prepared to tech and aware that he might throw it out if their are any gaps in your offense.