Well, assuming you'll be using it in Battle Spot Singles (Special Pokemon allowed; on the DS, not smogon), This set should work:
Naganadel @ Dragonium Z
Ability: Beast Boost
EVs: 76 SpA / 180 SpD / 252 Spe
IVs: 0 Atk
- Draco Meteor/Dragon Pulse
- Fire Blast/Flamethrower/Heat Wave
- Sludge Wave
- Nasty Plot
The most common threats in Battle Spot Singles (again, on the actual DS, not smogon) are special attackers, like Ultra Necrozma (DW), Magearna, Geninja, Arceus (Mostly Fairy), and Kyogre. Common Physical attackers are Mimickyu, Ultra Necrozma (DM), Cloyster, Aegislash, and Hippowdawn. Anything else is generally non competitive sets (although some are).
The extra Special defense allows you to take Geninja's attacks better (like Aqua Shuriken) and allows it to possibly survive Ice Beam:
252 SpA Protean Greninja Ice Beam vs. 0 HP / 180 SpD Naganadel: 260-308 (90.5 - 107.3%) -- 43.8% chance to OHKO
252 SpA Greninja Ice Beam vs. 0 HP / 180 SpD Naganadel: 174-206 (60.6 - 71.7%) -- guaranteed 2HKO
252 SpA Life Orb Greninja Ice Beam vs. 0 HP / 180 SpD Naganadel: 226-268 (78.7 - 93.3%) -- guaranteed 2HKO
It only cannot Survive Ash Greninja when it's battle bond activates (and holds a Life Orb), but most people give Greninja a Focus Sash anyways. Devastating Drake OHKOs Greninja.
So, essentially, switch if Greninja survived it with a sash. You'll also take damage from the other special attackers better, just that Greninja Should be the most common Pokemon, but that depends on who you are randomly matched with, and therefore can vary.
If you want doubles help, I have no clue.
Source: Experience; Damage Calculator
Hope I helped!