WARNING it is recommended that you do the Squirtle Special Encounter first as doing this one first disables the being able to still be riding tauros after you black out thus making it impossible to reach squirtle. Edit
Only Available after Restoring Reborn City
Get a data chip from 7th Street and go to the Wasteland Hideout (Poison Gym)
The Healing Machine should be flickering, insert the data chip in, a starly and budew should appear, make sure to heal at this station then go to route 1 (recommended with only 1 pokemon strong enough to take a bouffalant) defeat a bouffalant to ride a tauros and then battle a trainer or enough wild pokemon and lose.
You will appear riding the Tauros back at the Poison Gym, Lana will start screaming about what even is happening at this point, proceed to leave the house (DO NOT go back inside or the tauros will run away) now continue as show in the image, where it says '1' there will be a rock to use rock smash on. (Be sure to stay away from the trees because venipede will jump out and scare tauros away)
When you go inside the cave you will need to use Flash (found in the underground Railnet behind a wall you have to use rock smash on) follow the green arrows for beldum and the blue ones for a Bonus that I will show you later. (You will have to interact with object 1 and 2 for the entrance to reveal it self, also the numbers in the image can be hard to see sorry about that)
Continuing with the Beldum event proceed to the secret entrance behind the pillar, then up the stairs to the left and out the door, you will reach a new area as shown.
Object 1 is a boulder to use Strenght on, object 2 and 3 are rocks to mine, object 4 is a Focus Sach hidden and object 5 is another boulder to use strength on continue to follow the green arrows.
After you leave the cave you will reach this location.
Object 1 is a Nugget, Object 2 is TM94 Secret Power and Object 3 is an Ability Capsule, Continuing with the Beldum Event you will reach this area.
Follow the arrows and interact with the broken machine for a level 1 Beldum carrying Metal Coat.
After going into the secret entrance you will happen upon an area with 3 'doors' and a sunstone in the rock to the left.
- Door 1 leads to an area where you can insert an Odd Keystone (found mining rocks) to get a Spiritomb along with 2 rocks to mine, so you could save here and soft reset mine until you got an Odd Keystone.
- Door 2 leads to an area with the TM66 Payback
- Door 3 leads to an area where you first will have to fight 2 level 30 Trubish and a level 65 Garbodor named Mr.Biggles after defeating Mr.Biggles you'll have access to Tm09 Venoshock, a PULSE read for Garbodor and 3 rocks to mine 1 of which you'll have to use blast powder (that you can get from DeFacto if you joined the Aqua Gang or Break if you joined the Magma Gang) to access a Dawnstone