The right answer would to be to pick a different character if the snake knows the matchup (rob wins 55 - 45 against snakes who don't know the matchup, but loses 60 - 40 if they know it), but if you must use rob, I am pretty confident that I know the correct things to do more than every rob aside from holy.
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Camp with gyros and lazers - Snakes that know the matchup realize that snake cant out camp rob so they wont even try but you might get a couple lucky hits for some damage if you mix up your camping game. If you notice the snake is dashing powershielding your projectiles, run up and grab when hes around f tilt range. This mixup will work a couple times a set. If you notice they are walking powershielding your projectiles, you are gonna want to swat him away with f tilt. If they get inside ftilt range, you either need to bait an ftilt and punish according or run up and grab. The run up and grab works ALOT in this matchup because it is not expected. You should be d throwing every time unless you are close enough to the ledge that bthrow/fthrow will send him off.
Do not follow snake in the air - If you get snake in the air with d smash or d throw, charge gyros when hes recovering high and hit him with it as hes landing. If you don't have time to charge, punish his landing with a grab or bair (if you read the airdodge). The only time you should be trying to Uair or Nair snake is when hes using the cypher or on a platform.
Only use Bair when snakes on the ledge - Spam fast falled bairs (back facing the ledge) when snakes on the ledge. He has no safe options of getting back on stage without taking some sort of damage if you use bair. Every other move gives him options.
If you are in kill percent, DO NOT LEDGE HOP FAIR - This is probably the most important because this is where snakes get most of their kills on rob. They are expecting this because this is the most common way rob's get on stage from the ledge. Most of the time they will powershield and punish, and if they don't... at low percents fair isn't even safe on hit, snakes have time to up tilt. The only time you should be using fair against snake is retreating fairs (very very good for baiting snake dacus) or if you catch him with a fair string while they are using the cypher (even then, I think we only get 2 - 3 fairs before it stales too much). The safest way to get back on the stage is using the stages environment. Ledge hop backwards and Up B to a platform (On smashville wait till the platform is on your side, on BF fly up to the top platform, etc.). These aren't guaranteed safe ways to get on stage, but it's pretty much the best option aside from ledge hopping backwards and spacing the gyro as close to ledge as possible. I'm going to stress the point again DO NOT FAIR SNAKE AT ANY POINT IN THE MATCH ON STAGE UNLESS YOU ARE RETREATING. WE HAVE NO FOLLOW UPS AND AT NON HIGH PERCENTS, ITS NOT EVEN SAFE ON HIT.
Snake Ftilt - If snakes try to bait you with only using the first hit of ftilt on your shield, just roll away. Do not try to punish it, just roll away and keep camping. DO NOT TRY TO PUNISH IT.
Rob's Pseudo CG - I invented a pseudo CG that works against characters who don't have multiple jumps: Dthrow - Headbutt part of bair as they are landing - Grab
After you d throw, staying on the ground, you follow them while they are in the air. As soon as they are landing you headbutt them with bair, if they airdodge the bair you get a free grab (dthrow again). If they don't airdodge the bair, you can hit confirm into d smash or grab again. It take a while to get the spacing down, but once you do it's so godlike.
Killing Snake - Get lucky. No, seriously though. Get lucky. The only way we can kill snake is if he makes a mistake, take advantage but do not fish.
Snake's Dthrow - This is the most annoying part of the matchup. Good snakes are going to **** you up with this unless you get lucky. The most reliable option IMO is waiting a second or 2 after he d throws you and react. If he doesn't move, roll forward. If he tries to predict the forward roll, roll behind him. If you get grabbed on a ledge just put the controller down, its over.
Pummel - It's so important to pummel after every grab you get on snake after hes at ~50 percent. You NEED to have you bair refreshed because thats your only chance on getting an early[ish] kill on snake if he gets offstage. Try to keep Nair fresh until hes at like 130 too.
Platform Pressure - TICKLE SNAKES FEET. UP AIR ALL DAY BUT DONT OVER COMMIT OR HE CAN PUNISH IT ON SHIELD. TICKLE HIS FEEEEEEEEEEET. DONT USE FAIR, DONT USE BAIR, DONT USE NAIR. JUST TICKLE.
There's probably more that I am forgetting, haven't used rob seriously in a while