I'm not sure how many people know this, if it's relevant or not, but i was told a few days ago by Fatal that C4 lasts 54 seconds before it explodes automatically.
It's not THAT game changing but i think it's a nice tidbit of information and could really help with port advantage if you lead him into it with a grab. The C4 has much more knockback than grenades so the automatic explosion could be played in our favor, but it's a highly unlikely situation.
I have played Fatal and Bizkit multiple times, as well as other mid-level snakes from my region, and i really don't think he is in one of Marth's worst 3 match-ups. I hear a lot of speak about ROB being a pain (unfortunately i lack overall tournament experience to have any input on how hard ROB is), and MK and Dedede seem far worse than Snake. Snake is doable, if you're having trouble i would definitely recommend grabbing more in this matchup. It's been mentioned a few times, but grabs pretty much shut down all of snake's landing options. A well spaced pivot grab beats landing Bair, landing Nair, grenades obviously, and airdodges obviously. Pivot grabs work really well on snake's that recover high, which should be almost always, and the only thing that beats it are B reversals, which can be predicted or reacted upon, and C4 tricks.
I'm not sure how many people encounter it but I know Fatal likes to use this trick. When you hit snake in the air with an upsmash or an up air or up tilt, he will lay the C4 and then double jump, then explode the C4. If you try to react and think he will land you just eat the explosion. But most snakes do this that i've played. What he does differently is you dodge the C4 explosion, but then he lays another and explodes it right after. The point of this is you assume since he has no jumps left you can punish and run straight into the second C4. It's really hard to get around, but the best option is to predict it and try and hit him after he lays the C4 but before he explodes it. This way he can't use more C4 in the air, but is still in that same bad position. Either that, or don't chase him at all. The second C4 creates a wall between you and him, and most of the time he is the one escaping, and puts him near the ledge, which is still favorable to you.
It was mentioned in another thread (i forget who, but credit goes to them), the down variation of the 3rd hit of dancing blade stabs at their feet and then steps REALLY far back so you're out of grab range. If you find yourself attacking their shield too much with dancing blade and getting shield grabbed too much, this could be a nice mix-up to avoid the grab. I haven't tested this much, so i'm not entirely sure how well it would work, if at all.
Pay attention to your airdodge habits, snakes feed off of this. If you find yourself airdodging into the ground too much, they will catch on and eventually punish you every time for it.
I'm not really a knowledgeable player or that great in general, but I really enjoy the match-up and have played good snakes so i figured i would post something. But take this with a grain of salt, i suppose.