I like this team and feel that it's well rounded... however, I do have some suggestions!
Firstly, Araquanid. I love this Pokemon; it's underrated and highly powerful if used right. Araquanid has quite a remarkable SpD without the use of an Assault Vest. I suggest you use(or try) to put those EVs in HP into SpD, and the 4 EVs in SpD in Def, this way, your Araquanid will have fairly high Attack, Defense, and Special Defense... and in terms of Items, while I did use an Assault Vest on this Pokemon at one point, I've switched to a Life Orb (for a terrifyingly powerful Liquidation that is boosted by STAB, Water Bubble, and the Life Orb), then to a Cell Battery. Araquanid is already powerful, but the Cell Battery allows it to raise its Attack by 1 when hit by an Electric-type attack. This can also be substituted with a Weakness Policy since Rock-type attacks are fairly common in the Meta as well. If you decided to try these Items out, it could get rid of Crunch and give you room for a crucial move like Protect.
Next, Clefable. I have no qualms or anything to say here other than that that Flamethrower isn't going to hit hard since I'm running virtually the same set. You have T-Wave, which makes Flamethrower seem a little more useless with that 10% chance of Burn. I understand it's to get rid of Steel-types, but most Steel-types hit hard enough to destroy Clefable in one or two hits. I can only see Flamethrower being effective against Kartana if it doesn't outspeed and obliterate Clefable first. I suggest using Dazzling Gleam instead to get STAB and hit both opposing Pokemon. Wish is good if you're playing the long con game, but if you want to obliterate your foe as soon as you can, Moonlight would be the better option here. Lastly, its Ability. While I can see where you're coming from, the majority of Pokemon Trainers don't care about the genders of their Pokemon(unless you're me lol), and if they do, the chances of Infatuation occurring are 30%, and even so, a Pokemon that's Infatuated can still attack you, so it's all about pure chance here with Cute Charm. I suggest using Magic Guard since you won't be affected by anything unless it's an actual attack. Trust me, being afraid of Hail, Sandstorm, Toxic, Will-O-Wisp, etc. will be a thing of the past with a Magic Guard Clefable.
I think your Nihilego set is great, but with a 4x weakness to Ground-type attacks, an Air Balloon would be quite useful unless you are outsped and the Air Balloon is popped. Also, most Pokemon Trainers like to raise their Pokemon's stats, like the S-Dance Mimikyu, or Multi-hit attacks + Stamina Mudsdale, or Dancer + Quiver Dance users... I suggest you use Clear Smog to help you with that. Clear Smog, although not too hard-hitting with that Base Power of 50, is boosted by STAB and will clear those annoying stat raises right up. This is only to be taken into consideration if you wanna play it safe.
Kommo-o is looking good here! The Timid Nature, 252 Spe, and Choice Scarf make it quite fast. It's moves have great coverage, and I just wanna warn you that whenever a Dragon-type Pokemon comes into play, destroy it with Draco Meteor while you can and if you can(because it's Scarfed) because Dragon-type Pokemon. Hit. Hard.
Metagross is also looking good! Mega Metagross has been popular since ORAS, so I'm sure you know what you're doing here(sorry if I'm sounding high-and-mighty, just commending you). Oh, and since your team lacks any Protect, Earthquake will hurt them too, especially Nihilego and Bastiodon, so be cautious when Metagross is around them!
Lastly, we have Bastiodon. It looks pretty good here as well! If you want more than an Iron Head, you could potentially use Rock Slide instead. It hits both your foe's Pokemon and has STAB!
Sorry if this was lengthy lol
