I noticed Clef and Gar are weak to Bisharp, Mega Sciz, Talon, and Char-X. The cool thing is the core can cripple them via status. Because of this, I decided to begin with a standard CM Clef with wave. WoW Gar followed shortly after, being helpful to burn things like Bisharp and Mega Sciz, helping Clef out lots. Although this is how I began, I would later change CM Clef for reasons explained further down.
Status can cripple some of the threats mentioned, but that isn't enough. I wanted to choose a Pokemon capable of reliably handling most of them, especially Bisharp and Mega Sciz. Keld immediately came to mind, serving as a good offensive check to those two, while providing more speed and crucial resistances to keep things like Weavile at bay.
After picking Keld, I wanted a steel to stop Mega Altaria and Mega Gardevoir from being so threatening. Another check to Clef and Mega Venu wouldn't hurt either, especially with how often Gar is trapped. I didn't want to end up with a passive team, so I ended up choosing Mega Metagross for the role and to provide some firepower. In addition to checking the things I was looking for, Mega Gross takes advantage of Gar burning things like Mega Sciz, a nice bonus.
At this point, Char-X and Talon were still issues. I was also missing SR, and noticed Exca could sweep me. A bulky ground only seemed natural from here. Lando-T was the first thing to cross my mind to fill these gaps. I disliked its vulnerability to being burned by SpD Talon though, and picked SD Gliscor with Rock Slide instead. This accomplishes much of the same as Lando-T, while handling Talon better and being more durable with Roost. SD Glis also gave me with a win condition, which the team lacked. Additional answers to CM Mega Sableye and CM Clef couldn't hurt either. The only downside to this change was no SR. Because of this, I had to give my Clef rocks, which I didn't mind too much.
Only one slot left, and I noticed my switch ins to opposing Keld and other waters were lacking. I ended up going with AV Tangrowth, which can check Keld and Pokemon like Rotom-W decently. TG Manaphy with Ice Beam still troubles it, but through speed and para, the team is alright against it. Tangy's ability to check Mega Diancie is also appreciated, because Clef is constantly forced to heal by Moonblast, and no longer has CM to soften the blow. It's also a good pivot, which is useful if I need something to absorb an attack.
With all 6 mons, I looked for weaknesses and tried covering them with sets. One weakness I noticed was LO Serperior, so I gave Sludge Bomb to Tangy, which also gives me another option against Mega Gard. I also gave BP to Mega Gross. Between all of this and SR + LO recoil, Serp can be checked well enough. Despite having Keld, I noticed Bisharp and Mega Sciz still annoyed the team lots. So, I decided to make Keld a RestTalk variant for more durability. For more Bisharp coverage, I made Gliscor faster than Jolly variants. The other issue I noticed was Charizard-Y. This is hard to cover without changing lots, but I tried decreasing its opportunities to switch in by giving Tangy and Gliscor rock coverage. Gengar, Keldeo, and Mega Gross are also faster, while Clefable can paralyze, so the team does have some methods.
The other thing I could see as annoying is Skarm. The good thing is 3/6 of the team is immune to Spikes, and every mon except Mega Gross has a form of recovery. I considered Thunder Punch on Mega Gross for Skarm, but think BP for picking off Serp is too important.