The team:

@ Choice Scarf
Ability: Levitate
EV’s: 252 hp, 212 def, 44 speed
Bold Nature
IV’s: 31/16/31/31/31/31
- Trick
- Thunder Wave
- Flash
- Memento

@ Leftovers
Ability: Clear Body
EV’s: 252 hp, 100 atk, 146 def, 12 sp.def
Careful Nature
IV’s: 31/31/31/30/31/31
- Substitute
- Curse
- Amnesia
- Iron Head

@ Lum Berry
Ability: Sand Veil
EV’s: 252 atk, 252 speed, 4 def
Jolly Nature
IV’s: 31/31/31/31/31/31
- Substitute
- Swords Dance
- Outrage
- Earthquake
The team I use is very similar to Peterko’s team (and others using Registeel) except that I use Latios over Latias, so credits to him. Huge credits to Jumpman16 and Peterko in general for getting 2k+ streaks. That is absolutely insane, especially in the Platinum/HGSS Tower.
I prefer Latios over Latias because he's one of my favorite Pokémon, and I prefer looking at him rather than Latias for an extended period of time, which is what happens during Battle Tower attempts. As stated before, the team originally wasn’t meant to be my main team but I enjoyed it so I kept going with it. Using Latios gives me access to Memento, which I always been a fan of since it gives me more flexibility in setting up Garchomp on opponents locked into status moves or low PP moves. It also has the benefit of not having to risk switching in Garchomp and taking a crit from a "safe move". There are situations Charm + Latias’ slightly better bulk would come in handy, but overall the team has been working well! Latios has been given 44 speed EV’s in order to reach 136 speed, which is enough to outspeed Metagross4 after Trick.
Registeel and Garchomp are self-explanatory. Registeel sets up on everything that is able to break Garchomp’s Sub. If Registeel gets frozen I PP stall the ice move and will set up Garchomp on Struggle. I always want to try to have a sub up with Registeel when the 2nd Pokemon comes in. This can mean that I will try to make the lead Pokemon die to Struggle while I'm using Substitute in case it breaks my current sub with a Struggle crit. Garchomp is given 4 Defense EVs to achieve an odd HP stat, allowing it to use Substitute four times. If I have the opportunity I will try not to sacrifice Latios.
When I can I will try to set up Garchomp over Registeel vs PI trainers. This is because the 2nd Pokemon can be QC Donphan4 which can give Registeel trouble. For example when I face Pinsir lead and lock it into X-Scissor, I will count X-Scissor and PP stall, switch back to Latios on Struggle, then use Thunder Wave + Memento and set up Garchomp.
Threat list:
• Rhydon4: An opponent that has crushed many high streaks on the leaderboard. QC Horn Drill OHKO is almost a guaranteed defeat. Really hate
• Machamp3: if Machamp3 QC Ice Punch freezes me I'm dead. Really hate
• Abomasnow1-4: Hail is annoying, really limits me in setting up. Locking Abomasnow into Sheer Cold is also dangerous if he keeps hitting Garchomp’s sub. Hate
• Rapidash3: Leading with Trick is too risky, cause it outspeeds Garchomp and can be locked into Horn Drill. Best to lead with Thunder Wave, then switch to Registeel. If Latios dies to Horn Drill, switch in Garchomp.
• Kingler3: Leading with Trick is too risky, cause it outspeeds Garchomp and can be locked into Guillotine. Start with Thunder Wave. If Latios dies to Guillotine, burn Guillotine PP with Registeel. Kingler3 is dangerous, since it can break Registeel's sub with Crabhammer when Registeel's defense stat isn't raised.
• Gliscor3: I'm dead if it is locked into Guillotine and it hits 4 times in a row.
• Wailord4: switch between Registeel and Latios to burn Blizzard and Fissure/Hydro Pump PP.
• Lapras2: dangerous if it Quick Claw OHKO's Latios. Best to switch in Garchomp and try to use Substitute.
• Empoleon3: best to switch between Registeel and Latios and burn Blizzard PP because of Quick Claw.
• Salamence4/Garchomp4/Dragonite4: When locked into Outrage it can be risky if it crits Registeel. Immediately set up Sub with Registeel against them. Outrage does 11 hp damage at +6 Registeel defense
• Zapdos1-4: could be locked into U-turn which is annoying. I prefer to lead with Thunder Wave against it.
• Yanmega1-4: best to switch to Registeel immediately since it could be U-turn Yanmega2 which is annoying when it U-turns on Latios.
• Gastrodon4: switch between Registeel and Latios to burn Earth Power and Ice Beam PP. Then Flash x6 + Memento.
• Rampardos1-4: Probably best to lead out with Memento, since I've seen Rampardos4 use Earthquake against Latios due to Mold Breaker which is annoying. I try to outstall it with Registeel if it's locked into Head Smash. If it's Rampardos3 I will switch to Garchomp after Memento, then switch to Registeel to absorb Head Smash. Then switch between them to burn Head Smash PP and set up Garchomp.
• Staraptor4: Safer to use Trick over Memento since CB Staraptor hits like a truck with crit. Then set up with Registeel