All right--here is how I fight GOOD snakes and some to-do's and not to-do's
Advantages:
1) Grab out of mortar slide -- a lot of snakes like to get close, whether it's a good thing to do against Olimar or not. But it happens, so watch for it.
2) COUNTER PICK STAGES -- This is how I can keep up in tourneys with stages. There are 3 amazing counter-pick stages against snake as Olimar. I will go into detail below.
3) Combo's and juggling (sometimes juggling is possible)
4) Pikmin throwing on explosives/grenade spammers
5) Edge-guarding
Disadvantages:
1) Snake Tilts (obviously)
2) Easily edge-guarded
3) Invisible mines in dark levels (like Lylat--probably won't see if you missed the plant)
4) Being juggled/kept in air
Advantage details:
1) Grab out of Mortar Slide: Obviously this is the most known olimar vs. snake tactic, and can be used completely to your advantage in the matchup. Snake can keep you juggled if you allow him to get you in the air with this mortar slide...which is extremely annoying.
2) COUNTER PICK STAGES: This is the one big thing I could suggest to Olimar players vs. Snakes. Pick a good counter-pick stage! Here are 3 of them I have found to be quite...acceptable.
a) Norfair -- No more super sliding snake! haha! This isn't the only reason to pick this level vs. a snake either. Because he can keep you edgeguarded so well, you have a chance to land in rising lava once gimped. Not only that, but you can recover in THREE (yes 3) different sections on both sides of the stages. You can play extreme mind games with this as well. You can stand on the very top platform, SPAMMING pikmin as quickly and wait for the snake to approach, quickly hug, drop down to the bottom and jump up and u-air him into a juggle or combo. A good way to play against snake here is to spam pikmin and continuously run around (avoid a snake player if he stands on the bottom middle platform and waits for you--just keep throwing pikmin at him because he is very secure on that platform. This also calls for a longer match, but it doesn't have to be played that way.
b) Brinstar -- Brinstar is a very good counter-pick for snakes as well. You can ledge camp and jump under the level and u-air, but that can be a bit risky. You also have the multi-platform recovery options like Norfair, and the lava to help you. This also forces snake to move around--the lava. He can't just camp the bottom every moment of the match. But keep in mind, he can keep you off the top platform if the lava rises to it's full height. This matchup isn't as well as Norfair, but it half-way eliminates gimps. Snakes do well against Olimar because he is just so easy to kill.
c) Finally -- Luigi's Mansion: This can be an extremely well thought counter-pick against a snake if he bans Norfair. You can camp one of the bottom corners, which can be a large advantage for you. Snakes might try to mortar slide to get close to you, allowing you about 2 seconds to react and grab him. IF you can pull off a grab on him, throw him UPWARDS if he is not in the middle, this will throw him into the ceiling. He would then bounce downwards and lead to you multiply up-smashing him into ceiling, back down, again, etc. Really good players still usually get hit by at least 1-2 smashes before they tech it, but that's a 30-40 % damage increase on their sides. Another thing to watch for would be snakes starting to grenade camp you if you keep running to the bottom away from him. Easily dodge-able just by throwing pikmin to stop his grenades in mid-air, or by moving to the top platform (but don't let him get under you!
3) As far as combos and juggling go, there are plenty of responses to those just read. Keep in mind that up-air and white pikmin are good ways to rack up damage.
4) Obviously won't need to get into pikmin and explosions.
5) MOST IMPORTANT is your edge-guarding!!!
a) A LOT of people try to jump out there and try to spike him, because spike knocks him out of his parachute. Only do this when it is IN REACH of your first jump, or if they are are medium-close range. Don't get impatient and jump out if he's just going to parachute above you. Wait for him to get to the stage and up-air him, but watch out for his air dodge and punish. mind games can be used if you think he is going to air dodge your u-air. This is a game of patience vs. snake!
b) FAIR knocks out of his parachute when he is at higher percentage, and so do purple pikmin. Keep this in mind when you jump out to kill him. 85% of the time they will be too high or too far out to do any effect ledge guarding, so just camp the stage and wait for his up-b to finish.
c) The parachute grab. Just grab his parachute when he up-b's against the stage. As olimar's pikmin can grab people when they are on the stage, they can also grab his parachute lower than most people's grabs. keep that in mind. Watch out for the sneaky snakes though, they might let go of their parachute and hit you and then mine themselves back on the ledge or to get another up-b higher up so you can't grab his parachute. IF you predict this then you can anticipate the mine blowing up and prepair for a fair or a spike as he blows himself upwards.
As far as disadvantages go... don't be the fool to run into his tilts randomly. Space yourself away from him, run away if you have to and be prepared to grab him!
2) easily edge-guarded---this goes both ways. You can edge guard him, but he can edge guard you as well. BE PREPARED for the b-air, it's very large hit box and kills you pretty much instantly if hit when having no double jump. try to fair him unexpectedly before he can b-air or just air dodge. It's good to save whistle for this time to if you have good prediction timing.
3) mines--watch out on Lylat and other dark stages, mines can cause you to go above him into his dam juggling or even kill you. Pay attention to his moves and make sure you don't forget about the mines o.O
4) being juggled---it all comes down to watch out for his mortar slide. If he can mortar slide you above him it's hard to land if he stays below you. His up-tilt is rediculous and you can't just air dodge past it because when you hit the ground he can hit you before you can shield.
Anyways, this is all I have right now. It all comes down to patience and watching out for his quick moves like mortar slide and f-tilt. I hope this helps some of you, so good luck fighting those snakes!