Since the most recent change just a few days ago (hours, even), the quest to access Sinnoh has been completely replaced. If you were in the middle of the quest, about to start it or had beaten Hannah but not talked to her and teleported to Sinnoh, you'll have to start the new one from scratch. Good luck, trainer!
VERY BIG WARNING: ONCE YOU ENTER ROUTE 101 TO FIGHT THE GRUNTS, DO NOT, I REPEAT, DO NOT GO BACK TO LITTLEROOT TOWN. IF THE NPCS DISAPPEAR AND YOU CANNOT PROGRESS, POST HERE: https://pokemonrevolution.net/forum/topic/871-url . THERE SEEMS TO BE A BUG WHERE, IF YOU GO BACK TO THE TOWN AND THEN REENTER ROUTE 101, THE GRUNTS COMPLETELY DISAPPEAR AND YOU CANNOT CONTINUE THE QUEST UNTIL YOU ARE HELPED BY PRO STAFF.
WELCOME TO THE SINNOH ACCESS QUEST GUIDE!
The quest will start once you speak to the NPC Paul in Lilycove City (Hoenn). You need to have 6 pokemon in your team that are at the very least level 80 or above. This can be acheived by training them in Mount Silver or in Cerulean Cave as the NPC says. However, getting access to Cerulean Cave is painful, so we can also borrow a friend's team. The pokemon do not have to be ours to be valid for the quest, so we can borrow 6 pokemon at level 80 from a friend or train them ourselves.
He will then teleport us back to Littleroot Town. Once you exit your house, you'll find unwanted visitors: Team Galaxy Grunts! The villagers need help!
You will then approach Stan, who stares at you, menacingly (yes, this is a Jojo's reference). He will fight you with strong pokemon. He has a level 70 Exploud, Hariyama, Crobat and Purugly.
They will then Teleport and escape.
Go north to route 101. More grunts will appear. The next one has a level 70 Mightyena, Dustox, Masquerain and Luxray.
Grunt Bernie is next. He has a level 70 Mawile, Aggron, Electrode and Skuntank.
Grunt Jack comes afterwards. this will take long... He has a level 70 Cacturne, Altaria, Whiscash and Bronzong.
Grunt Mario has a level 70 Whiscash, Claydol, Banette, and Drapion. There's still more!
Grunt Karina has a level 70 Medicham, Dusclops, Glalie and Toxicroak.
The last Grunt, Vincent, has a level 70 Absol, Tropius, Flygon and Weavile.
You will then reach Oldale Town. Heal up and try to go to Route 102. Stan will block the way and fight you with his level 100 Alakazam.
When he's defeated, he will teleport you to Route 103.
You'll see Mars, Saturn, Pluton and Birch. However, you'll hear a mysterious voice: Maxie and Archie will take care of two of them, while you'll fight Pluton. He is a tough one.
Pluton has a level 100 Slowking who will spam Scald, a Rotom-Wash, a Porygon-Z, a Blaziken, a Sceptile and Swampert (the pokemon they forced to evolve, it seems).
Since many people asked for help on this step, I will give you an example strategy of how you can beat this boss easily:
1.- Geodude with Sturdy + Stealth Rock (or any evolution, you can capture one in Mt Silver with the moveset ready as Graveler)
2.- Electrode with Eerie Impulse (you can get one at a high level already in Cerulean Cave)
3.- 1 Pokemon with a good setup move. Examples: Gyarados with Dragon Dance, Clefable with Cosmic Power, Slowbro with Calm Mind and Iron Defense, etc. For this boss, I recommend you either unlock the Mega Slowbro Stone or you use a Clefable, easy to farm and use.
You begin by using Stealth Rock with Graveler and letting it faint. Then you Eerie Impulse multiple times against the Slowking and faint your Electrode.
After this, switch in your pokemon with a setup move equipped with leftovers, set up until your stats are maxed and you will be able to sweep. Example:
Clefable - Cosmic Power, Toxic, Softboiled, Moonblast
Gyarados - Dragon Dance, Earthquake, Waterfall, Ice Fang
Slowbro - Iron Defense, Calm Mind, Slack Off, Surf
However, they will escape after you defeat him. You will talk to Birch and he'll actually tell you that the three Hoenn starters are lost in Route 101 and 102.
Mudkip can be found just below Birch in the Route 103. You need Surf to pick him up here.
Torchic will be hiding in the spot in front of me in Route 101. Interact with it to rescue the lost pokémon.
Treecko will be hidden in front of me in this spot in Route 102. Interact with it as you did before.
Once you have picked up the three Pokemon, talk with Birch.
He will tell you to go and talk to Alakazam. He can be found in the Docks in Lilycove City (same place you can go and get the boat to go back to Vermillion and Olivine.
Congratulations, you've now completed the new Sinnoh Quest!
VERY BIG WARNING: ONCE YOU ENTER SINNOH, THERE IS NO GOING BACK. YOU MUST DEFEAT THE ELITE FOUR IF YOU WANT TO GO BACK TO THE PREVIOUS REGIONS.
Special thanks to:
@WhiteBaron (mr corsi handsome french boy) for helping me out and fixing the variables that bugged while I was doing the quest.
@Donuts02 for lending me his level 100s so I could rush Hoenn to do a guide of the quest for forums.
@jimmicha for rushing my account story in Silver so I could do more guides for forums.