Whats the Regigigas team? I'm curious!
It's this:
Regigigas @ Persim Berry
Ability: Slow Start
EVs: 156 HP, 100 Attack, 252 Speed
Nature: Jolly
-Return
-Drain Punch
-Knock Off
-Swagger
Xatu @ Focus Sash
Ability: Magic Bounce
EVs: 252 HP, 4 Sp.Def, 252 Speed
Nature: Timid
-Psychic
-Air Slash
-Tailwind
-Simple Beam
Registeel @ Quick Claw
Ability: Clear Body
EVs: 252 HP, 252 Attack, 4 Sp.Def
Nature: Adamant
-Iron Head
-Hammer Arm
-Rock Slide
-Psych Up
Totem Mimikyu @ Mimikium Z
Ability: Disguise
EVs: 252 Attack, 252 Speed, 4 HP
Nature: Adamant
-Play Rough
-Shadow Claw
-Shadow Sneak
-Psych Up
Essentially, I lead with Xatu and Regigigas. Xatu is naturally slower but moves first due to Slow Start, and Xatu will use Simple Beam on Regigigas to replace Slow Start with Simple. Regigigas in turn will use Swagger and target Xatu. Xatu will automatically use its ability to bounce Swagger back, and due to Regigigas' Simple ability (which Xatu gracefully provided) Regigigas will go to +4 attack. The confusion gets healed by the Persim Berry. After that it's setting up Tailwind or fainting for Xatu and either Registeel or Mimikyu come in and Psych Up the attack boost. It gives me so much offensive pressure I can just dominate the battle on sheer power and clean up from there. Regigigas has a lot of natural bulk amplified by Drain Punch healing (it easily survives Kartana's Bloom Doom for instance), Registeel has enormous bulk and hits very hard at +4, and Mimikyu has an easier setup thanks to Disguise and also priority access. Regigigas at +4, even without full investment and with a Jolly nature, OHKOs almost anything (Knock Off OHKOs Mega Metagross for instance). The power is that Regigigas is balanced around its Slow Start ability, but without Slow Start it is a pokémon with a higher base attack stat than Groudon, more bulk than Mega Kangaskhan, and as fast as a Charizard, and if you can quickly get that to +4 attack it is an absolute colossal monster.
The unreliability is in the setup. Xatu reflecting Swagger can miss because Swagger only has 85% accuracy, which delays the setup by a turn and by then I've given away the game to the opponent. Opponents focusing down Xatu before I can get Swagger off (Regigigas is slow on the first turn) also ruins it. The Focus Sash prevents that partially, but Xatu does go down to double-focusing.
I also have a fifth pokémon in the team right now, this Linoone:
Linoone @ Salac Berry
Ability: Gluttony
EVs: 148 HP, 4 Attack, 52 Defence, 52 Sp.Def, 252 Speed
Nature: Jolly
-Extreme Speed
-Belly Drum
-Simple Beam
-Swagger
This EV spread gives maximum mixed bulk after maximising speed while also keeping the HP stat even, with the 4 leftover EVs spent in attack. Linoone can essentially perform two roles. First it can Belly Drum into Salac Berry next to a Psych Up user like Mimikyu. Linoone is slightly faster so it can get Mimikyu to +6 attack, +1 speed on the first turn. It can then apply offensive pressure through Extreme Speed. The other role is as a backup Swagger user for Regigigas to boost it up if Xatu goes down too fast, but I'd have to sacrifice either Registeel or Mimikyu on the 4v4 team for it. Now that I think about it, I should probably also change the item on Registeel, don't want Quick Claw to proc Psych Up to activate before Linoone sets up.
I'm actually considering Terrakion for the 6th team member. Terrakion also gets Psych Up, which is this teams synergy, gets solid damage with Rock Slide and Sacred Sword, and also has Quick Guard so I could run a Linoone Terrakion opening if the opponent has Fake Out on their team, although the problem is that a lot of Fake Out users (like Salazzle and Swift Swim Ludicolo) are faster than Terrakion and Quick Guard only works against Fake Out if the Fake Out user is slower because they're in the same priority bracket. Tapu Bulu is also an option, it also gets Psych Up and has a lot of natural bulk possibilities (though not as much as Registeel) and can sustain itself with Horn Leech. Tapu Bulu also cancels out enemy terrain.