So I play Solharath quite a lot and have been making significant headway against him in the Snake vs. Sonic matchup, so I feel like I can help a lot here:
1. The stage pick is somewhat solid: a small stage limits the high mobility of Sonic, and the single platform takes away some of his combo game.
2. You need to think about what environment Sonic thrives in and try to limit that environment: that is to say, use a lot more down smash. It limits Sonic's movement options even further and forces him into the air more often (which is where Snake wants him) or it forces him to roll and air dodge to detonate mines (which Snake can punish).
3. The tranq punish at 0:40 was severely lacking. First, Sonic is directly in front of you, so the C4 stick is absolutely free. Second the grab lacked pummels, which are invaluable for racking up quick damage. Third, you actually can lead into grab with single hit dair for another free 4% (but that's a bit cheeky). Fourth, I think at that percent, u-tilt -> c4 actually has a strong chance of KOing Sonic outright (that can also be led into with single hit dair or nair). If not, a single down throw tech chase into regrab will lead to a KO with u-throw -> c4 (with pummels for cushion). Fifth, your grab clearly meant to throw sonic into your mine above; the problem with that is that it was so telegraphed and obvious that your opponent easily DI'd to avoid it (it wouldn't have KO'd anyway). A Tranq hit confirm should lead into a kill at that percent, easily. At the very least, you could've gotten a good edgeguard situation out of it.
4. Learn to DI Sonic's attacks. The DI on Sonic's throw at 0:49 looks just wretched. You're lucky he didn't follow up on it at all. That could've easily been a fair spike (potentially into your mine).
5. The second tranq hit confirm was fantastic because he was at a percent where fair would've easily KO'd. Good on ya there.
6. Why did you Cypher during Sonic's invincibility? Keep your options open in those circumstances, otherwise you're liable to get comboed to death. You might've also wanted to refill tranq, considering you used two on the previous stock.
7. 1:10 is the second time I've seen you randomly fair. It's really questionable given the long cooldown of fair. Act wisely in the air.
8. That super high recovery at 1:15 was really nerve-racking for me as well. Sonic has really good vertical mobility and great moves to abuse that fact (namely, u-air and fair). Don't be above Sonic. You didn't get punished for it, which gives me the idea that this Sonic doesn't exactly know his character's tools that well.
9. The grenade you dropped afterward was a good thing, even better that your opponent blundered into it and missed the tech. But you didn't capitalize on that: the second you saw him jump, you should've been considering a tech chase scenario. I would've dair'd there to jab reset the no tech and still be able to follow most tech options. You also could've just wavedashed in and responded the reaction. Instead you whiff the bair and the grab and Sonic gets away. Then you spotdodge and grab again, neither of which makes any sense.
10. 1:34 sees you hitting yourself with a grenade again, which means you need to practice your grenade cooking more so that you're familiar with the timing.
11. The second tranq of the game also goes uncapitalized, but this one is less your fault: Sonic is relatively low percent and a follow-up is tough. Personally, I wouldn't have even tranq'd here, but a clean DACUS probably would've hit sonic and allowed for a u-air string or an opportunity to set up a downsmash and establish stage control. You're also out of shots here.
12. 1:42 was a really nice idea, you just got outread. Good show. Remember how you were out of shots? No, you forgot! and you die for it.
13. Then waste two more on your invincibility! Set up your stage control!!
14. Shieldgrab needs to be done sparingly: it only works when your opponent attacks your shield unsafely. Sonic spaced his fair really well, and while I don't know the frame data, it's clear that this instance is one where it's unpunishable.
15. The third tranq of the game leaves a lot to be desired. It was also a hard read (which paid off this time) that I would avoid repeating: had the sonic rolled inward, they'd have rolled through the tranq with invincibility and been able to punish your cooldown. Your punish also could've been better as they're again at the percent of the first tranq hit. They're a bit further away but you still could've gotten a lot better than a soft hit fair.
16. Your "edgeguard" is really lack luster as well. The mine is really far into the stage, and then you fail to put yourself in a position to punish Sonic's recovery option. Sonic has only a few options now:
a. Spring jump, which fair and bair are really good about punishing
b. Spring jump into dair, which the mine tends to cover pretty well, so I would assume he'd be afraid of doing that.
c. Sweet spot the ledge, in which case you just grab ledge and force something else, typically something laggy or poorly spaced to force them into your mine and launch them.
d. Homing attack. This isn't nearly as common now due to the 3.5 nerf, but if you just air dodge, Sonic goes into special fall and dies.
You also don't have Sonic stuck here, which means you can use your C4 to cover even more space. It's tough to do on reaction, but it's something you can definitely get into the habit of doing.
17. The u-throw -> mine gimmick from before worked. Examine what was different about the two circumstances and take note of it. I'd attribue this one's success to just one thing: it's lower on the platform than before and any DI was bad DI because of it, although it looks like he failed to DI it based on the angle the u-throw sent vs. the angle the mine sent. You got kinda lucky there.
18. The back-throw to fair gimp doesn't work very well at low percents, and even then, it's better to run off double jump rising fair instead. It allows you to wait out your opponent's jump and then catch them, which has the benefits of better accuracy and better change of KOing because they can't use their DJ any more. Snake's recovery is really high, so you should be able to make it back from any attempt: just be wary of when your opponent baits you into going super low—some characters are better at recovering than Snake.
19. Your use of C4 is actually rather frustrating. It started out okay, but your opponent quickly caught on to the gimmick and got around it. The c4 detonation is pretty slow and is definitely prone to reaction because of it. Something as simple as a roll or spotdodge will avoid it, let alone Sonic's long foxtrot.
20. You ran out of tranqs again because you wasted so many before. Realize the fault of this going far beyond that fully charged f-smash.
21. You're relying too heavily on shield grenade now as well. Sonic is just waiting out the explosion and then hitting you during the shield stun: he gets you multiple times by doing this and eventually combos you into a fair spike off of it.
22. A fourth tranq (if you remember, I said each tranq shot should result in a KO, so you should've already won at this point) and you jump into your own mine! Again, bad time to tranq, and DACUS is far better. Come to think of it, I haven't seen you DACUS once. Learn to DACUS pls. It's super good.
23. A FIFTH TRANQ. It trades with down smash and you get sent off stage. The benefit here is that they can't edgeguard, but you pull a grenade anyway (an anti-edgeguard tactic) at very close range (it won't cook long enough to be of use here) which then puts you into an edgeguard situation where Sonic is actually awake to edgeguard you. He goes for a super risky fair when he could've just punished your on stage Cypher (bad idea, just sweetspot) and still hits you with it because of that one bad grenade.
24. A sixth tranq, and his DI was good, but your fair was also really bad which made it seem like no big deal really. Get the tipper fair spacing down: it KOs ~30% earlier.
25. You wavebounce cypher by accident here, which shows you're fidgeting a bit on your controller. Calm down, take things slow, and focus a bit more. This leads to a really rough edgeguarding situation where you manage to C4 damage boost a lot (kudos). Be more aware of your options: the first one was entirely too high (you get fair'd for it). The second one was perfect—but all you needed to do with walltech into turnaround cypher and grab ledge. You also hardthrow a grenade when you really needed to normal throw it. work on that. You do eventually land back on stage with cypher (eeeeeeeh) and then sit in your shield and let Sonic pressure you back off stage and into a fair KO. In that situation, you should've been prepared to hit confirm off of Cypher instead of sitting in your shield. A u-tilt would've covered Sonic's descent, as well as Cypher OOS to unite the two options together some.
26. I'll point it out again: DI SONIC'S THROWS. I was positive a fair spike was gonna come out after that bad DI at 4:38.
General takeaways:
1. Learn your tranq setups better. This means both hitting and comboing. Catching opponents with it in neutral only works once (typically). Use it in tech chases and to catch laggy moves; that's it. When you DO get the tranq, capitalize on it the best that you can. Your priorities ought to be:
A. C4 stick
B. Launch for u-air juggle and/or C4 KO, whether it be u-throw, u-tilt, d-tilt, or DACUS
C. Set up for an edgeguard (via tipper fair)
D. Set up for a tech chase (via downthrow)
DON'T let them return to neutral without a cool 50%+
2. Utilize C4 sticks. Sonic didn't get stuck once this game. Sonic dies really easily to C4 (he dies the eleventh earliest based on my findings). You do this by forcing Sonic into situations where he's predictable: when he's sitting in shield afraid of a followup, when he's asleep due to tranq, when he's flying out of a u-throw, when they're using ledge invincibility to destroy your mines, et. al.
3. Tighten up your recovery. Snake is a one trick pony in this regard: even with all the damage boost options, he has to Cypher eventually, and Cypher is punishable in all situations, but it's least punishable when you sweetspot. My visual aid is to press down just as the machine peaks above the stage: Snake's grab box was actually raised slightly in 3.5, so it's far safer to sweetspot now.
4. Tighten up your edgeguards. Sonic isn't an easy character to edgeguard, but he shouldn't be getting a free pass on stage either. You've gotta set up situations where you'll know how Sonic will get on stage and then put yourself in prime position to punish that option. This is done by controlling their movement with mines and C4.
5. Learn your combos. Snake can combo the piss out of nearly every character. Sonic is rather floaty, so ease up on your fairs and start using nair, dair, and the weak hits of bair and u-air. Grab a lot more and capitalize off of them. Snake's grabs are amazing combo starters.
6 Learn to DACUS. Learn to DACUS. Learn to DACUS. Learn to DACUS. Learn to DACUS. Learn to DACUS.
