This achievement is story based and should be completed during natural progression. Beneath the Meridian is a main mission that chronologically occurs as below:
Children of the Vault
From the Ground Up
Cult Following
Taking Flight
Sanctuary
Hostile Takeover
The Impending Storm
Space-Laser Tag
Atlas, At Last
Beneath the MeridianEssentially, start by traveling back up to Sanctuary and handing the final fragment of the Vault Key to Tannis in her lab. Report now to the bridge to finalize the plan on looting the vault and fast travel back to Meridian Metroplex. Grab a vehicle and make your way NE with some reinforcements which include Zero. Take out the baddies along the way so Zero can lower the shield letting you enter Neon Arterial.
Before heading on, grab the Typhon Log on the left overlooking the water. You'll meet up with Maya next to the Fast Travel and jump aboard Project DD courtesy of Ellie. Follow the relatively straight forward roads to Apollyon Station fighting/fleeing bandit vehicles along the way. Maya will also attack any that get close to you. There will be two instances in which you must clear out the area of baddies so Maya can open the roadblocks. Eventually, you'll end up at the train station where you can load up.
Work your way into the station through a big battle (there are some good chests in the container tower on the left side) and grab the 2nd Typhon Log on the near right side from where you entered. Once Maya blasts open the door, you''ll fight through several rooms with waves of enemies pouring in at you. You'll get to the tracks in which a train will unload more bandits at you and then the train will occasionally pass on through so be sure not to get hit by it. You'll also see a Claptrap on the right side of the tracks.
After passing out of the station, you'll descend into a tunnel where you'll meet your first guardian. You'll be battling these guys the rest of the way just bear in mind they can move quickly so keep some distance. After you pass a waterfall coming from the ceiling there is a slight branch on the left with the last Typhon Log. Since this is the 3rd one, the drop will appear on the other small branch from the main cave just a little but further up on the right. After this, you'll have one more big fight before entering The Forgotten Basilica.
Make sure you gear up now and clear your backpack because it's time to fight the Vault Monster - The Rampager. Slide down the ramp into the basilica and activate the pedestal near Maya then brace yourself. Some tips for this fight is to simply keep moving to avoid his projectiles, fire at him when he's up high and unleash everything when he is stunned. There will be occasional guardians popping up to help as well as these flying spheres that protect him so make sure to address these as well. When he's finally down, grab the loot and head into the vault. Head straight to the icon and grab the Eridian Resonator which will allow you to finally smash those eridium crystals you've seen around. Do so on the chests here and grab everything else before heading out.
You'll meet up with Ava and Maya and trigger a cut scene where
*** Spoiler - click to reveal ***Tyreen and Troy will appear and ambush your group. Tyreen will grab Ava and Maya takes Troy as collateral. Troy grabs Maya which unknowingly absorbs her power, disintegrates and kills her. The twins will take off so talk to Ava and head back to Sanctuary. After a speech from Tannis and Lilith on the bridge, follow and talk to Ava before turning in the quest which unlocks the achievement.
The achievement should pop and you'll be rewarded with:
- Approx $2,904
- Approx 11,408 XP
- Eridium x2
- Weapon Slot
- Cloud Kill SMG (if Ava is spoken to)