Ok so with this decision coming up it is very interesting and an important suspect to start off this new meta in my opinion. From my experience, Naganadel is a pokemon that can either do very little in a match because of the matchup it is put into, or it can straight up sweep teams. For example, when Naganadel goes against teams with scarf greninja or pursuit trap alolan muk (Dark & Poison). In these cases, Naganadel would be lucky to get one or two kills and that's usually normal for a mon with such offensive presence. However, as I stated before, this mon can sweep teams with the set that is getting it suspected in the first place, dragonium nasty plot is just way too much for this meta. With Naganadel being able to sweep water on its own if scarf greninja is not on the team to revenge kill with ice beam, and if the water user is against poison, naganadel just switches out into toxapex and comes back in later to clean up the team. Also, this is one of the few special attacking mons that can ohko a chansey with its z move, it isn't the first because waterium z manaphy also can do this with it's tail glow set. However, there are ways for normal to stop Naganadel from sweeping with the likes of banded diggersby with its priority, Lopunny's fakeout to get chip damage on it if it gets whittled, and ditto itself. Let's not also forget about the steel MU because this mon can nasty plot up on ferrothorn and gets access to fire coverage to pressure that type as well, keep in mind it took specially defensive heatran to check this mon in OU, which I don't think max spD is ran on steel or fire for that matter, there will be calcs below to show how threatening Naganadel can be offensively, and things you may not realize defensively. So that being said, with a mon that can either do nothing, or sweep teams because there simply aren't enough counters to it in general; should it be banned or not? Well, I believe that it does over centralize the meta currently and is one of the best breakers in monotype right now, and even though it does have a predictable move set; it doesn't matter because even if you know the set, how do you switch into it? Using the monotype philosophy this mon is very good for poison, I believe poison really does like having this mon, and so does dragon because having something to finally pressure the p2 + chansey core on its own is really good, but is it too good?
Calcs:
252 SpA Naganadel Devastating Drake (195 BP) vs. 252 HP / 16+ SpD Mantine: 211-249 (56.4 - 66.5%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Naganadel Draco Meteor vs. 252 HP / 16+ SpD Mantine: 141-166 (37.7 - 44.3%) -- guaranteed 3HKO after Leftovers recovery
+2 252 SpA Naganadel Draco Meteor vs. 252 HP / 16+ SpD Mantine: 280-331 (74.8 - 88.5%) -- guaranteed 2HKO after Leftovers recovery
This gen mantine has found it's place on both water and flying, being good in defensive cores with zapdos and celesteela on flying, and with toxapex and swampert on water, this is their mixed wall because of mantine's amazing spD. However, naganadel abuses this mon and if you're toxic mantine, you cant toxic this poison type and if you're haze, it has too much offensive pressure to stop it from ko'ing mantine or forcing the switch.
+2 252 SpA Naganadel Draco Meteor vs. 252 HP / 4 SpD Toxapex: 307-363 (100.9 - 119.4%) -- guaranteed OHKO
252 SpA Naganadel Devastating Drake (195 BP) vs. 252 HP / 4 SpD Toxapex: 231-273 (75.9 - 89.8%) -- guaranteed 2HKO after Black Sludge recovery
252 SpA Naganadel Draco Meteor vs. 252 HP / 4 SpD Toxapex: 154-183 (50.6 - 60.1%) -- 85.9% chance to 2HKO after Black Sludge recovery
Toxapex is another defensive mon that has come into monotype in gen 7 that has become very good in its longevity because of its regenerator ability and has provided a great toxic spiker for both types, it has made both types good at pressuring types that cant handle their mons getting toxic'd. However, Nagandel beats down and sets up on toxapex with ease, regardless of haze or not. I believe phys def toxapex is the best set to run in this meta cause of how much it's able to wall for its respected types, and naganadel abuses it. Both mantine and toxapex are known for being good haze mons and Naganadel has proven to beat both with ease.
+2 252 SpA Naganadel Devastating Drake (195 BP) vs. 252 HP / 0 SpD Eviolite Porygon2: 436-514 (116.5 - 137.4%) -- guaranteed OHKO
0 SpA Porygon2 Ice Beam vs. 0 HP / 0 SpD Naganadel: 176-208 (61.3 - 72.4%) -- guaranteed 2HKO
Moving on, naganadel also abuses normal. Now people may be wondering why a person would use their z on porygon2 and it's simple, porygon 2 walls dragonite once it traces dragonites multiscale. Dragonite itself does very good at breaking through the type if it's dragon dance, I can see staraptor also stopping dragonite from sweeping, but dragon does very good at setting up rocks vs normal; but i would preserve the Z if I was using nagandel on poison because once pex gets tspikes up, p2 loses to it regardless. Now, I am aware that porygon 2 can run download, but you should put ur 4 left over ev's to put it into SpD to avoid this situation and resulting in you breaking through porygon2.
0 Atk Ferrothorn Gyro Ball (150 BP) vs. 0 HP / 0 Def Naganadel: 199-235 (69.3 - 81.8%) -- guaranteed 2HKO
Ferrothorn doesn't always carry gyro ball, occasionally it does carry power whip instead to have a shot vs water with hazard stacking teams to pressure the type. This calc shows that Naganadel should always be able to get 1 nasty plot for free versus this type and can force heatran in to pressure it. Keep in mind, Naganadel isn't in this matchup to sweep, it helps both poison and dragon break through tran to have a better shot of breaking through the type because of how good it is defensively vs both.
+2 252 SpA Naganadel Devastating Drake (195 BP) vs. 248 HP / 136+ SpD Heatran: 238-281 (61.8 - 72.9%) -- guaranteed 2HKO after Leftovers recovery
Pretty straight forward calc, this is a more defensive tran set that would be used to check a mon like Naganadel, I will also show the calc for max hp tran with no spD
+2 252 SpA Naganadel Devastating Drake (195 BP) vs. 252 HP / 0 SpD Heatran: 297-351 (76.9 - 90.9%) -- guaranteed 2HKO
This is also straight forward, this damage is insane and the fact that a resisted move like this takes away at least 60% of your hp from both of these spreads makes it very hard for heatran to stop dragon from using its fire coverage because it is getting whittled down so much by this mon because of other passive mons letting naganadel set up on it
252+ Atk Choice Band Huge Power Diggersby Quick Attack vs. 0 HP / 0 Def Naganadel: 165-195 (57.4 - 67.9%) -- guaranteed 2HKO
This calc is important because it just goes to show that even though Naganadel has a poor defensive stat of 73, a mon as strong as diggersby struggles to pick it off and that naganadel will slowly break through these bulky offensive builds.
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 0 HP / 0 Def Naganadel: 205-242 (71.4 - 84.3%) -- guaranteed 2HKO
I do not have much to say about this calc, I'm not going to say that naganadel always revenge kills because you can run different hp spreads on azumarill and light screen can be active if it is against fairy and azu gets its belly drum off with them up or not, this calc is more relevant for dragon, but its also irrelevant because a lot of dragon types take aqua from azu at +6. This calc can matter in one instance for poison if its the azumarill set which carries return/frustration to hurt venusaur, but that set doesn't run adamant so that is also something to consider.
The Conclusion
I believe that Naganadel is a bitter sweet pokemon in this meta. It punishes people who use bulkier or stall type teams, making it easier for players to pick a type to use and keep under control. However, it also punishes offensive types because there just are flat out no good switch ins because even some the bulkiest mons in the meta can't take it on and prevent it from setting up. It is great when you have it on your side, and annoying to play around when you're against it because if your check gets whittled down, even 2 stealth rock switchins can lose you the game. Naganadel is just too good for this meta and I will be voting ban.