There are no "hard counters" to Gliscor. A counter is a Pokemon that can consistently switch in against another Pokemon and threaten death every single time. A hard counter is that same concept but just amped up right past the stratosphere. An example of a hard counter is flash fire Heatran being the biggest middle finger to Volcarona. Volc cannot do sht to Heatran no matter what thus Heatran is a hard counter. A regular counter would be Toxapex against Zard Y. Pex completely walls Zard Y even with solarbeam and stall out sun turns and can make Zard suffer a slow and painful death. Of course, this isn't counting national dex because if that was the case, I think Zard actually gets scorching sands so, rest in pieces Toxapex. It is also important to note that everything I mentioned thus far goes out the window with terastalization. This mechanic is a dual middle finger to the concept of counters and just destroys matchups. Take that Heatran and Volcarona example. Before, Heatran shts on Volcarona, now Volc gives it the middle finger, teras water or ground, and deletes Heatran from the game
Now, as for what I said earlier, Gliscor quite literally has no counters. You might think that Corviknight would counter it but in actuality, it doesn't. It's a stalemate. Gliscor can't kill Corviknight but Corviknight can't kill Gliscor either. Now, the reason Gliscor has no counters is because it is one of the few Pokemon that still retains toxic, meaning switching into it is not as easy and Pokemon that are immune to toxic are steel and poison types, which Gliscor destroys. One thing that might come to mind is cm Clefable. Magic guard makes it immune to toxic so it can cm in its face and eventually break thru with moonblast but Clef is having a difficult time right now. Garganacl could also be a strong contender because salt cure and toxic heal negate each other therefore any chip it deals to Gliscor will stick. Even resisted body press will not be healed thanks to salt cure and of course, this monolith is immune to status. Though, you have to burn your tera because sodium chloride is a rock type
What you're looking for instead are "checks", Pokemon that can beat another Pokemon but cannot safely switch in. To defensive Gliscor sets, what you're really looking for are Pokemon that hit it super effectively like Waterpon, Greninja, Weavile, ice spinner Tusk, Wake, just to name a few. Tusk doesn't instantly kill Gliscor but if it catches Gliscor on the switch with spinner then it's in trouble. As you see, none of these guys love the idea of switching in to toxic or losing their items. Oh, I should probably mention Gliscor still has knock off, which makes the idea of switching into it even more difficult. Aside from Clef which I mentioned earlier, and I suppose Garganacl, nothing really wants to eat a toxic or lose its item. Defensive Gliscor sets usually run a combination of protect and earthquake, then two among spikes, tspikes, knock off, you turn, stealth rock, and toxic. You can even look at the viability rankings and see how the things that destroy Gliscor don't really want to switch into it at all
What I just mentioned is for defensive Gliscor. Offensive Gliscor is another pain in the ass because it runs sd with facade, eq, and sd. Sometimes it even goes agility just for the shts and giggles. This thing gets walled by Dondozo but that's about it. It's really strong if it gets going and it's still Gliscor, meaning it's always gonna be obnoxious to kill. You really needs Pokemon that can take a +2 facade or eq and strike back on its weaker special defense to even stand a chance and that list isn't exactly long at all. Maybe Hatterene or a full health Ursuluna BM
Of course, none of those things I mentioned take into account tera, which Gliscor can abuse. On defensive sets, water is the most common tera type because it shrugs off its ice weakness without sacrificing too much defensive utility. On offensive sets, it's usually normal so that it gets stab on facade. Keep this on the back of your mind when facing this damn scorpion
I also have a couple fun mons to deal with Gliscor. The first is hex Dragapult and the reason should be obvious. The second, and the more fun one, is neutralizing gas galarian Wheezing. Watch as Gliscor almost instantly dies to its own toxic. You do have to be careful because Wheezing is a poison type but if you can safely switch it in, Gliscor just loses a sht ton of health. Wheezing isn't even that bad because it is the only defogger that can defog against Gholdengo. Make no mistake, it's not ideal but it does have a good niche and really, it makes Gliscor really sad
