Working on overhauling my own Singles team, and I have 3 members so far.
Salamence @ Salamencite
Ability: Intimidate
Adamant Nature
252 Atk / 252 Spe / 4 HP
- Double-Edge
- Earthquake
- Roost
- Dragon Dance
Toxapex @ Black Sludge
Ability: Regenerator
Calm Nature
252 HP / 148 Def / 108 Sp.Def
- Scald
- Toxic
- Recover
- Haze
Marowak-A @ Thick Club
Adamant Nature
252 HP / 252 Atk / 4 Sp.Def
- Flare Blitz
- Shadow Bone
- Bonemerang
- Flame Charge
I still have 3 more spots to fill, and I'm not sure what else I should be covering. I don't know what I should be wary of and what to prepare for.
I've been told to build a "core". That's something I should figure out so I can complete one.
As it exists right now, at first glance, your team is obnoxiously weak to Mamoswine, doesn't have any solid counterplay for Jolly Salamence(which now is something that is acceptable to run thanks to the mega speed change), has very little ways of dealing with any Tapu Lele set meaningfully, and keels over and dies to most rain cores (when i say "most rain cores", i mean Kingdra in rain alone can take it)
When people mean cores, they often mean 2 or 3 mons that cover each other very well, such as Pelipper/Kingdra, Tapu Bulu/Aegislash, or in a case particularly relevant to your team, Celesteela/Marowak.
Celesteela, depending on what sets you decide to use on it, can clear up any of these weaknesses nicely. I'm not entirely sure you need an anti-wall SubSeeding set thanks to the Toxic, so this is a set that may help just generally.
Celesteela @ Leftovers
Ability: Beast Boost
Level: 50
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Leech Seed
- Protect
- Heavy Slam
- Earthquake
Additionally, if you don't feel squeamish using Celesteela with Electric Terrain up over half the time, you could attempt to use either Mamoswine or Tapu Koko, both of whom synergize pretty gosh-darn well with Mega Salamence offensively.
Mamoswine @ Focus Sash
Ability: Oblivious
Level: 50
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Icicle Spear
- Ice Shard
- Earthquake
- Stealth Rock
Tapu Koko @ Life Orb
Ability: Electric Surge
Level: 50
EVs: 4 HP / 4 Def / 248 SpA / 252 Spe
Timid Nature
IVs: 1 Atk / 30 SpA / 30 SpD
- Volt Switch
- Dazzling Gleam
- Grass Knot
- Hidden Power [Ground]
These are just suggestions though. If you were to try this team out with my three sets, you'd be hard-pressed to not bring a relatively passive Toxapex as your only counterplay to Cloyster, for example. I have no idea what replaces Toxapex, or if Toxapex is even an okay mon regardless of what teammates it is given. I hope this helped!