Pros:
+73/127/121 is fantastic offensive stats, and beast boost only makes it scarier
+Poison/Dragon is a near-unique typing that grants fantastic coverage and isn't weak to fairy types
+Fantastic movepool containing Key Poison/Dragon/Fire coverage to hit everything for neutral bar Heatran
+Access to nasty plot got this thing banned
+First pokemon added mid-generation
Cons:
-73/73/73 defenses is low by OU standards, and awful by ubers standards, even with a large number of resistances
-Poison/Dragon cant touch steel types at all, forcing it to run fire type coverage
-Nasty weaknesses to dragon, ground, ice and psychic
Ability:
Beast boost: Boosts Naganadels highest stat when it scored a KO. With proper EV investment and a correct nature, Naganadel can gain speed boosts instead of special attack boosts. Combined with Nasty plot, thing thing can snowball out of control after a single nasty plot boost. No wonder this thing got banned.
Sets
So nasty it got banned to Ubers
Item: Dragonium-Z
Nature: Timid
Ev's: 252Spa/4SpDef/252Spe
Moves:
-Nasty Plot
-Sluge wave
-Draco Meteor
-Fire Blast
Fire/Dragon/Poison coverage hits everything except for heatran, but after a Nasty plot boost even Heatran won't want to stick around into a +2 Devastating drake. Poison coverage nails any fairies who think they can come in freely on this thing, and Fire blast hits steel types that resist Naganadels STAB moves. Unfortunately, low bulk makes Naganadel weaks to being revenge killed by the likes of Marshadow or Mega Lucario. Focus sash is an alternative if you want that survivability, but without a Z-crystal Naganadel will struggle to break though certain walls.
Other Options:
Naganadel has a plethora of other coverage moves it can run, including thunderbolt, air slash, dark pulse and signal beam. Unfortunately, running any of these over fire blast or flamethrower means Naganadel gets walled by steel types including Necrozma-Dusk and Ferrothorn.
VGC 2018
Item: Focus sash/Life Orb/Z-crystal (Poison or dragon)
Nature: Timid
Ev's: 4Hp/252Spa/252Spe
Moves:
-Tailwind
-Sludge Bomb
-Draco Meteor/Flamethrower/Heat Wave
-Protect
Naganadel is a lot less impressive in doubles, but thanks to tailwind it can certainly pull its own weight on your team. Single-target attacks are the way to go, so sludge bomb is run over sludge wave, despite lower power. Due to the abundance of fairies in the format, including the 4 tapus as well as Mega gardevoir and Azumarill, Dragon-type coverage isn't strictly necessary. Fire-type coverage however, is due to the high number of steel types in the format. Protect is a staple for doubles.
Other Options:
Hidden power ground can be run to hit Heatran, while hidden power Ice hits Landorus-T. Thunderbolt hits Tapu Fini, Politoed, Gyarados and Azumarill, although sludge bomb already hits Tapu fini and Azumarill anyways thanks to their fairy typing. Dark pulse hits psychic types, but most of the important psychic types come with a secondary fairy typing, which again, is taken care of by Sludge bomb. Helping Hand is an interesting choice that can make an ally's attack hit like a bus. However, Naganadel already hits relatively hard, so it would make more sense for an ally to use helping hand on Naganadel instead.
Partners:
Since Naganadel can handle the Tapu's exceptionally well, anyone who appreciates their removal makes for a good teammate. This includes almost any fighting type or pokemon that wants to spread status conditions, such as Rotom-Wash, who fears Tapu Bulu and can't spread will-o-wisp while Tapu fini's Misty terrain is on the field. Thanks to tailwind, any team mate with a less-than-impressive speed stat can start sweeping. This includes Megazard-Y, who also appreciates removal of Tapu fini and Tapu Koko.
Naganadel itself wants partners that can remove its biggest threats, namely Heatran, and any threatening ground or steel type that can capitalize on its nasty ground weakness. Partners like Tapu Fini or Landorus-T threaten Heatran. Tapu Bulu's grassy terrain weakens Earthquakes that might other wise OHKO Naganadel, and Grassy surge removes Tapu Fini's misty terrain, which would weaken Naganadel's dragon type attacks. If you don't want to run tailwind on Nagandel, Suicune makes for an excellent Tailwind setter, and Naganadel counters any grass type that could threaten Suicune.