I'd like to present an argument in favor of removing Head Smash. Whether or not we go through with this nerf, we must go over this possibility's reasoning and consequences. I'll first go over the motivations of undergoing this nerf, and then present calcs to back up those claims.



Motivations for the removal Head Smash:

First, it addresses the most broken element on Mega Crucibelle, recoilless Head Smash. I've thrown around the term "power ceiling" on Discord, and I've realized that the term apparently...isn't the most intuitive. Thus, I'd like to elucidate what I mean by it. A Pokemon's power ceiling is how much damage it can deal out with its strongest attacks that it most commonly runs. So, Mega Crucibelle's power ceiling is most certainly defined by Head Smash, as it can outright OHKO and 2HKO the majority of the metagame. Gunk Shot definitely does too, but not to the degree that Head Smash does. Now, removing Head Smash would lower Mega Crucibelle's power ceiling quite dramatically. But why is this the problem I choose to focus on? Think about why U-turn is so effective. U-turn is effective when the user can force switches. Thus, the Pokemon's power ceiling should correlate with how well it can use U-turn. If Mega Crucibelle can't force as many switches, it doesn't get as many successful U-turns off of checks. It's also how Mega Crucibelle generates free turns to get Stealth Rock so consistently.



This is important because we can say that we can decrease a few stats here and there and call it a day, but if Mega Crucibelle still has the ability to force as many switches as it does now, how well are we really nerfing it? Sure, we can make sure that it takes residual damage with Rock Head, but it still has the ability to dish out huge volumes of damage. If we settle on it taking residual damage, the metagame will ultimately center around chipping down Mega Crucibelle with Stealth Rock, Spikes, Leech Seed, etc. I'm not sure if this achieves the level of balance we want for Mega Crucibelle.



Second, removing Head Smash preserves Mega Crucibelle's current niche in the metagame. Without Head Smash, Mega Crucibelle would have to rely on Stone Edge, but a set consisting of Stone Edge / Gunk Shot / U-turn / Stealth Rock is much different than the sets that would arise if we, say, removed U-turn. I find this important because Mega Crucibelle's niche is definitely an interesting niche: a strong, bulky, and fast pivot that sets Stealth Rock. In fact, it's basically the perfect niche. I do think have the distinct niche of a bulky and fast pivot (read, less strong) is important though, because if Mega Crucibelle loses its niche as a pivot, it faces intense competition from other Mega Pokemon like Mega Medicham, Mega Alakazam, and Mega Mawile, who are arguably better wallbreakers, especially if we got rid of U-turn. Thus, lowering Mega Crucibelle's overall damage output through removing Head Smash allows it to compete with other Mega Pokemon simply by occupying a different niche. As it is now, it blatently outclasses them because it reaches a power ceiling as similar to theirs while also having the bulk and speed attached to it.



Now, I'd like to look into the drop in power from Head Smash to Stone Edge. To put it in perspective, think about the damage output of Choice Band Weavile. Now, think about how much its power drops when its Choice Band is knocked off. That's the same drop in power that Mega Crucibelle would face by having to run Stone Edge. However, this allows a lot of breathing room for many of Mega Crucibelle's would-be checks! Most of them are worn down extremely quickly by Head Smash, but definitely not as fast with Stone Edge. Take a look:



Calcs:

Celesteela

252 Atk Crucibelle-Mega Head Smash vs. 248 HP / 28 Def Celesteela: 238-282 (59.9 - 71%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery

252 Atk Crucibelle-Mega Stone Edge vs. 248 HP / 28 Def Celesteela: 160-189 (40.3 - 47.6%) -- 3.1% chance to 2HKO after Stealth Rock and Leftovers recovery

Some Celesteela can elect to run more physically defensive sets, but it should be apparent how different Celesteela's matchup is affected by Stone Edge Mega Crucibelle. While it's not the best check, it can more reliably switch in, not be overwhelmed by Mega Crucibelle, and threaten to KO with Heavy Slam



Landorus-T

-1 252 Atk Crucibelle-Mega Head Smash vs. 0 HP / 4 Def Landorus-Therian: 183-216 (57.3 - 67.7%) -- guaranteed 2HKO after Stealth Rock

-1 252 Atk Crucibelle-Mega Stone Edge vs. 0 HP / 4 Def Landorus-Therian: 123-145 (38.5 - 45.4%) -- 14.1% chance to 2HKO after Stealth Rock

With Landorus-T's Z-Fly set, its best set right now, Landorus-T can soft check Mega Crucibelle. Again, it can be overwhelmed, but it's definitely not as easy to do so.



-1 252 Atk Crucibelle-Mega Head Smash vs. 252 HP / 112+ Def Landorus-Therian: 148-175 (38.7 - 45.8%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery

-1 252 Atk Crucibelle-Mega Stone Edge vs. 252 HP / 112+ Def Landorus-Therian: 100-118 (26.1 - 30.8%) -- 99.9% chance to 4HKO after Stealth Rock and Leftovers recovery

Defensive Landorus-T has a much better matchup.



Magearna

252 Atk Crucibelle-Mega Head Smash vs. 0 HP / 0 Def Magearna: 111-132 (36.8 - 43.8%) -- guaranteed 3HKO after Stealth Rock

252 Atk Crucibelle-Mega Stone Edge vs. 0 HP / 0 Def Magearna: 75-88 (24.9 - 29.2%) -- guaranteed 4HKO after Stealth Rock

Magearna, a Steel-type, can actually set up against Stone Edge Mega Crucibelle. Otherwise, it doesn't have time to get the necessary boosts to break past Mega Crucibelle with Focus Blast.



252 Atk Crucibelle-Mega Head Smash vs. 248 HP / 92 Def Magearna: 102-121 (28 - 33.3%) -- 0% chance to 3HKO

252 Atk Crucibelle-Mega Head Smash vs. 248 HP / 92 Def Magearna: 102-121 (28 - 33.3%) -- 28.9% chance to 3HKO after Stealth Rock

252 Atk Crucibelle-Mega Stone Edge vs. 248 HP / 92 Def Magearna: 69-81 (19 - 22.3%) -- possible 5HKO

252 Atk Crucibelle-Mega Stone Edge vs. 248 HP / 92 Def Magearna: 69-81 (19 - 22.3%) -- guaranteed 5HKO after Stealth Rock

The difference is all too apparent with the Pain Split set. Magearna can use this set to pretty reliably check Mega Crucibelle, especially with Pain Split.



Fidgit

252 Atk Crucibelle-Mega Head Smash vs. 4 HP / 0 Def Fidgit: 117-138 (35.2 - 41.5%) -- guaranteed 3HKO after Stealth Rock

252 Atk Crucibelle-Mega Stone Edge vs. 4 HP / 0 Def Fidgit: 78-93 (23.4 - 28%) -- guaranteed 4HKO after Stealth Rock

This calc is deceptively useful. With Stone Edge, now Mega Crucibelle is much more likely to put pivot Fidgit into Iapapa Berry range.



Mega Mawile

252 Atk Crucibelle-Mega Head Smash vs. 92 HP / 0 Def Mawile-Mega: 104-123 (39.3 - 46.5%) -- guaranteed 3HKO after Stealth Rock

252 Atk Crucibelle-Mega Stone Edge vs. 92 HP / 0 Def Mawile-Mega: 69-82 (26.1 - 31%) -- guaranteed 4HKO after Stealth Rock

-1 252 Atk Crucibelle-Mega Stone Edge vs. 92 HP / 0 Def Mawile-Mega: 46-55 (17.4 - 20.8%) -- possible 5HKO after Stealth Rock

Especially factoring in the pre-mega Intimidate drop, Mega Mawile can more reliably check Mega Crucibelle.



Kartana

252 Atk Crucibelle-Mega Head Smash vs. 4 HP / 0 Def Kartana: 100-118 (38.4 - 45.3%) -- guaranteed 3HKO after Stealth Rock

252 Atk Crucibelle-Mega Stone Edge vs. 4 HP / 0 Def Kartana: 67-79 (25.7 - 30.3%) -- guaranteed 4HKO after Stealth Rock

I'm blowing the same horn, but look at how much better this Steel-type with good, but not great, physical bulk can check Mega Crucibelle.



Naviathan

252 Atk Crucibelle-Mega Head Smash vs. 0 HP / 0 Def Naviathan: 138-162 (39.7 - 46.6%) -- 32% chance to 2HKO after Stealth Rock and burn damage

252 Atk Crucibelle-Mega Stone Edge vs. 0 HP / 0 Def Naviathan: 92-108 (26.5 - 31.1%) -- 97.1% chance to 3HKO after Stealth Rock and burn damage

Alright, Low Kick aside, Stone Edge Mega Crucibelle can't inhibit Naviathan's setup with its standard U-turn + Rocks set. Meaning, it has to go out of its way to run Low Kick to beat Naviathan.



Garchomp

252 Atk Crucibelle-Mega Head Smash vs. 4 HP / 0 Def Garchomp: 131-155 (36.5 - 43.2%) -- guaranteed 3HKO after Stealth Rock

252 Atk Crucibelle-Mega Stone Edge vs. 4 HP / 0 Def Garchomp: 88-104 (24.5 - 29%) -- guaranteed 4HKO after Stealth Rock

Again. Garchomp, anatural check to Mega Crucibelle, running an offensive set (SD Rockium Z) like it wants to actually putting up a good chance.



Ferrothorn

252 Atk Crucibelle-Mega Head Smash vs. 252 HP / 24 Def Ferrothorn: 97-115 (27.5 - 32.6%) -- 99.8% chance to 4HKO after Stealth Rock and Leftovers recovery

252 Atk Crucibelle-Mega Stone Edge vs. 252 HP / 24 Def Ferrothorn: 65-77 (18.4 - 21.8%) -- possible 6HKO after Stealth Rock and Leftovers recovery

Low Kick aside, this means that Mega Crucibelle has a much lower chance of hitting Ferrothorn on the switch-in and then following up with another hit and knocking out, further solidifying Ferrothorn as a check to non-Low Kick sets.



Toxapex

252 Atk Crucibelle-Mega Head Smash vs. 252 HP / 4 Def Toxapex: 175-207 (57.5 - 68%) -- guaranteed 2HKO after Stealth Rock and Black Sludge recovery
252 Atk Crucibelle-Mega Stone Edge vs. 252 HP / 4 Def Toxapex: 117-138 (38.4 - 45.3%) -- guaranteed 3HKO after Stealth Rock and Black Sludge recovery

While it's still a 3HKO, this gives Toxapex more opportunities to fish for a Scald burn and to recover stall out Stone Edge.



There are some notable guaranteed OHKOs that Mega Crucibelle loses out on:

252 Atk Crucibelle-Mega Head Smash vs. 0 HP / 0 Def Aurumoth: 510-600 (141.2 - 166.2%) -- guaranteed OHKO
252 Atk Crucibelle-Mega Stone Edge vs. 0 HP / 0 Def Aurumoth: 338-402 (93.6 - 111.3%) -- guaranteed OHKO after Stealth Rock



252 Atk Crucibelle-Mega Head Smash vs. 248 HP / 240+ Def Zapdos: 408-482 (106.5 - 125.8%) -- guaranteed OHKO

252 Atk Crucibelle-Mega Stone Edge vs. 248 HP / 240+ Def Zapdos: 272-324 (71 - 84.5%) -- 75% chance to OHKO after Stealth Rock and Leftovers recovery



252 Atk Crucibelle-Mega Head Smash vs. 4 HP / 0 Def Pajantom: 334-394 (107.7 - 127%) -- guaranteed OHKO

252 Atk Crucibelle-Mega Stone Edge vs. 4 HP / 0 Def Pajantom: 223-264 (71.9 - 85.1%) -- guaranteed 2HKO after Stealth Rock



However, I think that these calcs show how Mega Crucibelle can overwhelm a lot of its would-be checks with Head Smash AND without even considering its coverage moves, Low Kick and Wood Hammer. Even if we allow it to be chipped down by hazards with Rock Head, none of these Head Smash calcs go away. Thus, if we really want to look at what's making Mega Crucibelle broken/unhealthy, this is it.



To summarize,



1. With Mega Crucibelle's power ceiling lowered, it's much, much easily to check Mega Crucibelle defensively. We directly address why Mega Crucibelle is broken.

2. The above calcs support point 1.

3. By keeping what differentiates Mega Crucibelle from other Mega Pokemon, it can continue to compete with them but not completely overshadow them.