My interest in playing mons is unfortunately tied to a wide array of arbitrary factors like moon phase and ambient temperature so I don't have a lot of stake in how the nerf goes rn, but as the person most successfully gaslight into running PL and CL I do feel it's within my interest to chime in about my own observations on Cres, and what we can do to make the metagame better for those who are active.
I believe there are only four things we should be scrutinizing about Cresceidon's design. This isn't a flawless Pokemon, it's a fairly underwhelming damage dealer and occasional momentum sink to the point that people still run Prim in spite of how stacked our kit is. The issues mostly stem from the fact that trying to find a Pokemon that (A) safely switches into Cresceidon, and then (B) safely clicks a move into Cresceidon, is extremely difficult for a number of structures – mainly offense, but even slower builds can find themselves at a bit of a loss against the more demonic utility sets. Our best course of action from here is increasing the number of Pokemon that feel reasonably comfortable in front of Cres, namely, those which are more suited to faster offenses.
1. Thunder Wave
This is the big one and easily the most effective way to bump Cres down a notch or two. Maybe it's a bit beyond me to say this but it really feels that the longer you play and watch competitive mons, the more you come to understand that Thunder Wave (and Para in general) is often the best move one can make when it's affordable. Speed is such an important stat in how individual mons are meant to function, which then goes on to influence how whole teams synergize between members, so the fact that this shit gets paired with a cool 25% chance to do fuckall each turn with NO LIMIT really puts emphasis on keeping your team safe from Para as much as possible. Cresceidon is a fantastic user of Thunder Wave thanks to it's Speed and favorable matchup against most Grounds, and additionally finds itself in an environment low in splashable Electrics that would love to use a TWave Water as an entry point. This means the yellow magic button finds a lot of opportunities to get used during a match, and the more the opponent is paralyzed, the more Cresceidon and its teammates are able to push advantage. That 25% full para chance is especially nasty, as the Cres user will often decide that trading their own health for para is worth it, but because Cres outspeeds so much, there's a real chance that the opponent has their turn revoked by para, and Cres can go completely unpunished.
Now I want to preface all this with the fact that a fast, bulky Water, which is neutral to Ground and carries Thunder Wave, is an extremely strong and practical design for the kind of Pokemon we envisioned Cresceidon to be. Paralysis is very pro-concept for a fast wall: oh, something outspeeds you and removes your main advantage compared to other walls? Not anymore! It honestly comes across as so natural for Cresceidon's whole idea that I feel it'd be a shame to leave it behind. The blueprint I described above is not anywhere close to being overbearing on its own, Thunder Wave is only problematic because Cres has a lot of other, also problematic things it does, but this is still by far the most problematic thing Cres does in a vacuum, just by nature of Paralysis being Paralysis. We need to accept that axing TWave is the most straightforward path possible even if it feels integral to Cresceidon's design.
2. Encore
This is where things get real stupid. The number of Pokemon which can handle TWave Cres is rather modest, but definitely workable, and improves significantly when you factor in Tera or lures like Lum Berry. Encore presents a completely different form of danger but with ultimately similar purpose: do not let the opponent play the game. Cresceidon gets tons of opportunities to use Encore, given its Speed and great defenses.
You can stop setup users from getting anywhere, lock Pokemon into exploitable attacks, and bully bulky mons into wasting their limited Recovery on 16% HP or just sit there clicking Stealth Rock into the void. The mere threat of Encore generates situations where the opponent needs to respect the possibility, and the Cresceidon user is at a notable advantage here given that Cres, generally, does not die to any one attack and likely outpaces the switch-in, so it's more than happy to make the obvious play if it means the opponent now has to do something else.
Encore differs from Thunder Wave in being more prediction reliant, but having far greater immediacy in shutting down a threat and giving a teammate room to come in. If you're willing to drop a STAB, combining both moves onto a single set allows Cres to make some frankly disgusting plays. If you were to Recover with Arghonaut and Cresceidon comes in the logical conclusion is that it wants to Encore you, so the smart play is switching out, but now whatever you brought in better be safe against its STABs or Thunder Wave because you need to be prepped for all three cases. End result of this is that a lot of Cresceidon's safest answers are bulky mons with good recovery and status resistance, things like Snaelstrom, SpDef Gliscor, Garganacl (esp with Tera for the last two), and some tertiary switch-ins like Mollux, Glowking, Shox. It's not impossible to address, but the Pokemon listed definitely feel more at home on fatter structures which contributes to the dominance of bulky teams in CAP, and only those first three are truly safe from all of Cresceidon's immediate BS.
Something else worth mentioning: Encore largely prevents Substitute from being a good catch-all answer to Cresceidon. This mon has mediocre offenses with its primary Water move only scary because of the 30% Burn chance, and its other best button fails vs Substitute, so having a mon behind a Sub is extremely strong against Cres right up until it reveals Encore, and now we're back to square one. Without Encore, Substitute attackers are extremely good at frustrating Cresceidon's efforts.
Going forward I do not think we leave this process without one of Thunder Wave or Encore getting cut. When you only have the one it's annoying but good responses certainly exist; when you have to be prepared for the possibility of both, maybe even at the same time, approaching Cresceidon feels like you're constantly fighting on the backfoot, which is terrible for teams that lack the high sustainability needed.
3. Scald
A step below the previous two but not to the point where I feel it isn't worth mentioning. Given the tools at Cresceidon's disposal already, throwing in a STAB move that broadly pressures every physical attacker to get the hell out of dodge is just nasty. You know how much Caribolt would love to hard switch into Cresceidon? A strong physical attacker that you can't Thunder Wave and threatens Cres with both STABs meaning the response needs to either stomach both or guess correctly. Sadly you cannot take this risk because if it Scalds, you now face the very real possibility that Caribolt is ruined for the rest of the match beyond Healing Wish support. Even Pokemon that don't mind the halved physical damage aren't going to be happy that they are getting chipped every turn: bulky Gholdengo would be a surefire check to Cresceidon but the relationship gets significantly more shaky if it's Burned and now needs to spend more time healing, maybe you can utilize Covert Cloak to fully blank its options (also improves the Garg mu), but you're giving up other strong items like Boots, Lefties, or Balloon. It's just this extra layer of misery you need to weigh and I find it very unnecessary overall.
Removing Scald doesn't do enough on its own to make Cres more reasonable – again, I think either Thunder Wave or Encore have to go if we want to consider a nerf effective, but there's a strong case for taking Scald away as well.
4. The Speed Stat
This is a fairly understated aspect I find, however we do need to consider that the Pokemon which is infamous for bullying Offense does so primarily because it also outspeeds much of Offense. Now, part of Cresceidon's whole identity is that it's a wall with a high Speed stat. Our approach to this wasn't a wall that just so happens to outpace other Pokemon that don't invest in Speed, or are slow enough to not outpace us even with max investment. Instead we deliberately made a fully-fledged wall that tops off at 381 Speed, letting it blow past a number of already fast attackers. For refence, with the definition of "fast" getting more diluted each gen as power levels increase, I've always benchmarked a "fast" Pokemon as anything that can at least tie Wellspring's 350, given how dangerous she is as an offensive powerhouse. So yeah, Cresceidon's pretty damn fast, which leaves a rather short list of offensive Pokemon which can get the jump on it. There's 12 Pokemon which naturally outspeed Cres, of those only half are even good in CAP (those being Dragapult, Darkrai, Zamazenta, Stratagem, Deoxys-S, and Weavile), and of those remaining six, do any of them even want to face Cresceidon head-on? They certainly can with the right set or positioning or the funny Tera button but in a strict 1v1 it's definitely favoring Cresceidon. We are thankfully in the presence of other non-setup Speed options like Proto/Quark mons, various Priority attackers, maybe even a Scarfer here or there, so it's not completely unrealistic to have Cresceidon fighting uphill, but it's certainly much harder compared to literally every other wall in the game.
Maybe it's counter-intuitive to hit the brakes on a CAP designed to be fast, and I would agree that it feels somewhat disingenuous. At the same time I would also agree that we may have gone a good bit overboard with the Speed Stat, and personally as long as we're always outpacing the item-locked Woger I'd argue the concept is still being fulfilled, though I understand many others won't feel the same way.
What about Moonblast? Taunt? Abilities?
Yeah Moonblast is a dumb move and the SpA drop is annoying as hell, but it's significantly less of a long-term pain compared to status or losing all momentum from Encore. Both it and Scald were given because they work towards the "things a Fast wall does better than a slow one" as lowering your opponent's offenses before they even attack is great for walling. If we went full nuclear and stripped Cresceidon of any practical Status options Moonblast isn't going to be causing anything more than a mild headache.
Taunt has been mentioned as an option and some may feel it'll just replace Encore if we remove the latter, allowing Cres to continue harassing opponents just as it does now. While they are somewhat similar is dissuading setup or utility moves, their overall function couldn't be more different. Taunt requires you to pre-empt your opponent's options while Encore active abuses them. Taunt may prevent a SD user from boosting, but Encore prevents said SD user from using those boosts at all, while granting free entry for teammate all the while if desired. Taunt lacks that immediate pressure Encore presents in forcing numerous switches. This isn't to say Taunt doesn't have uses, it's still a strong move that appreciates users with great Speed, but the opportunities Taunt creates are far below the ones Encore regularly achieves.
Both Multiscale and Rough Skin are fine imo, Multiscale definitely makes this thing a nightmare to remove at times but that same property provides the metagame with an incredible pressure valve against SV's laundry list of scary offensive threats, and such a niche lets Cresceidon find a ton of value across various team structures. Being hard to KO isn't what makes Cres so oppressive, it's what it does while not being KO'd that people are displeased with. Rough Skin isn't as important to the design philosophy as Multiscale is but provides an avenue for Cres to have more immediate value against teams at the expense of significantly worse staying power, and that's a fair tradeoff in my eyes.
TL:DR the best route is going to be tackling either Thunder Wave or Encore, with considerations going to cutting Scald and some Speed as well. If I had to pick I'd prefer just killing Encore but I'm not super familiar with CAP atm so don't take that as gospel or anything, Thunder Wave still deserves getting axed simply as a bottom 5 game mechanic even if it plays into Cresceidon's concept much better.
