I'm not particularly good at formatting on this site, so I'll just walk through how I came up with this team.
I wanted to run two things: a stall team, and a Mega Aggron team. Luckily, the two go together. With my primary member chosen, I continued.
The first Pokemon that came to mind for stall was Forretress. Good defensive typing, all three hazards, and Rapid Spin. It can't do much to stuff that comes in on it, but the whole point of it is to set up hazards, then be brought out to spin when needed or to take a hit for the rest of the team.
I'd also heard about Assault Vest Metagross, so I put one on this team. It's got a decent defensive typing still, despite the loss of Dark and Ghost resists, and Assault Vest compensates for its softer Sp. Defense.
For Aggron, I originally tried a Rock Polish set. Then I discovered that Aggron's speed CANNOT be saved, no matter what. It provided another powerful physical wall with absurd defense and a solid movepool.
Originally, I'd intended to run a standard Blissey on this team to compliment my strong physical defense. I'd been walled to hell by Eviolite Chansey, though, so I used it instead. So far, it's worked out well, though I've had better luck with Blissey.
At this point team members were getting hard to choose. I wanted to run Skarmory for phazing Fairies, but I had already built up a massive Fire-type weakness and plenty of physical walling. I thought of a certain underappreciated Forme: Deoxys-D. While its health may be lacking, it has Stealth Rock and Recover, making it excellent for stalling.
My last choice was tough. I wanted another Rapid Spinner with access to hazards, so I picked Cloyster. I've since changed to a more typical Shell Smash set, but it has served me well, though I regret only having one spinner.
I've also considered swapping something for Milotic. One, Milotic has Dragon Tail, giving much-needed phazing. Two, I really like Milotic. Three, it makes a fairly good mixed wall and status absorber with Marvel Scale.
@ Leftovers
Trait: Sturdy
EVs: 252 HP / 252 Def
Impish Nature (+Def, -SAtk)
- Spikes
- Toxic Spikes
- Stealth Rock
- Rapid Spin
I usually lead with Forretress because it has good defenses and Stealth Rock. It also partially counters other hazard-setting leads, as it can set up its own then Rapid Spin their hazards away. However, it can't really dent anything, which has been a problem.
@ Eviolite
Trait: Natural Cure
EVs: 252 HP / 252 Def
Bold Nature (+Def, -Atk)
- Heal Bell
- Toxic
- Wish
- Seismic Toss
Standard Chansey. Works fairly well so far, and I invested in its Defense to make the best use of the 50% boost to both defensive stats.
@ Assault Vest
Trait: Clear Body
EVs: 252 HP / 64 Atk / 64 Def / 128 SDef
Adamant Nature (+Atk, -SAtk)
- Brick Break
- Meteor Mash
- Earthquake
- Zen Headbutt
This one is giving me trouble. Metagross works fairly well, but the EVs are killing me. I'm not sure whether to invest in Attack (since it was the only real offensive threat on my team) or Defense/Sp. Defense to tank more hits.
@ 306ite
Trait: Sturdy
EVs: 252 HP / 64 Atk / 128 SDef
Adamant Nature (+Atk, -SAtk)
- Earthquake
- Heavy Slam
- Stone Edge
- Dragon Tail
I still haven't decided what to use that first slot for. Rock Polish is out, Autotomize is even further out, but I feel like I need plenty of Stealth Rock setters on this team. Defoggers are a thing, and stall relies on entry hazards to deal damage to switch-ins. Dragon Tail uses a good Attack stat to dent things before phazing them out, and Aggron's Heavy Slam is absolutely monstrous. Stone Edge gives good coverage, despite the shaky accuracy.
@ Leftovers
Trait: Pressure
EVs: 252 HP / 128 Def / 128 SDef
Calm Nature (+SDef, -Atk)
- Stealth Rock
- Spikes
- Recover
- Psychic
I know, lots of rocks. I've explained this already. Deoxys's Pressure wears down valuable moves, Recover allows it to stay in on almost everything, and Psychic stops it from being complete Taunt bait, though so far the damage dealt has been worthless. I might swap it for Seismic Toss or, more likely, Night Shade, for reliable damage.
@ Leftovers
Trait: Skill Link
EVs: 252 HP / 128 Atk / 128 Def
Adamant Nature (+Atk, -SAtk)
- Rapid Spin
- Icicle Spear
- Rock Blast
- Shell Smash
I sacrificed some of Cloyster's base 128 defense to make it more of an offensive thread. I have Rapid Spin over Razor Shell to provide me with an alternative to Forretress, who is quite likely to die before Cloyster comes out.
Threats:Alakazam-Blaziken-Breloom-Chandelure-Chansey-Cloyster-Conkeldurr-Ditto-Dragonite-Espeon-Ferrothorn-Garchomp-Genesect-Gengar-Gliscor-Gyarados-Haxorus-Heatran-Hydreigon-Jellicent-Jirachi-Jolteon-Latios-Lucario-Machamp-Metagross-Mienshao-Porygon2-Reuniclus-Rotom-W-Salamence-Scizor-Scrafty-Serperior-Skarmory-Slowbro-Tentacruel-Terrakion-Thundurus-Togekiss-Tyranitar-Volcarona-Whimsicott-Zapdos-Celebi-Darmanitan-Heracross-Mandibuzz-Wuagsire-Sableye-Sigilyph-Smeargle-Starmie-Suicune-Toxicroak-Virizion-Hippowdon+Excadrill-Tyranitar+Landorus-Ninetales+Venusaur-Abomasnow+Walrein-Politoed+Kingdra-Politoed+Ludicolo-Politoed+Vaporeon-Chansey+Skarmory-Scizor+Rotom-W-Ferrothorn+Jellicent-