ALTDEUS Beyond Chronos: Achievement & Ending Guide

You can unlock two endings in the first Playthrough.
Here is a Map of the parts. Some can only be unlocked in further playthroughs.

Defeat the first Meteora
You can’t miss it, even if you miss a shot, you can still try again

Defeat the Meteora that killed Coco
It’s the basic requirement to get to the branch for any of the Humanoid Meteora’s Endings.

Purple Star is “Good” Ending, White Star is Bad Ending

View the “Astray from the Polaris” ending
This is Meteora’s Coco good ending. Select options that consider Humanoid Meteora as Coco. Follow Dieter’s plan.

Shot by traitorous Deiter.
Bad Ending. Chose options that disregard the Humanoid Meteora, you can be friendly with Noa. The trigger is the refusal to do Dieter’s plan.

Learns about the Makhia’s Analyze ability.
 
It’s a little hard to understand, through the screenshots, but the first image is almost complete. It’s the first half-star to the left on the second Screenshot.

Analyze the attacking Meteora on the surface
Self-explanatory, instead of killing, Analyze the first Meteora’s attack. Required for the 2nd+ branches.

Attend Noa’s night concert
This is guaranteed as long as you analyze the first Meteora from the above Achievement to unlock the new Branch.

Analyze the Meteora that killed Coco.
Similar to the previous Meteora attack, Analyze instead of attacking to open the 2nd+ branch.

Start exploring the surface.
Likely guaranteed once you enter the 2nd branch.

Fight alongside the Meteora with Coco’s form.
Guaranteed with the 2nd+ branch storyline.

Began researching the Unique Meteora Anima.
Likely Guaranteed as part of the story.

Attend Noa’s impromptu concert.
Guaranteed, Noa’s 2nd Concert soon after the experiments start.

Very easy to miss, I recommend you doing it first… For the sake of mood (select the Inhibitor both times during experiments on Anima to ensure. Better do it now than later, right?)

The Blue Star is the “Good” Ending, the white star is the Bad Ending.
 

Conspire with Noa about the elimination of the Unique Meteora Anima.
Be nice to Noa (Partnership+ options), and treat Anima as an evil Meteora. You entered Noa’s Path

Defeated the mysterious being. (This should be wrong… Defeated by would be more appropriate)
Noa’s Bad Ending. Don’t take Noa’s hand.

View the “The Fairy’s Ark” ending.
Noa’s “Good” Ending, take her hand when she offers it.

The Red star is the “Good” Ending, the dead-end is the Bad Ending

Decide to continue researching the Unique Meteora Anima.
Anima path. Be nice to Anima during the experiments and options related to her being similar to Coco.

Defeated Yamato in battle.
Guaranteed on Anima’s “good” ending…

Viewed the “The Celestial Cradle” ending.
Anima’s “Good” Ending. Select options to be nice to Anima, and then defeat Yamato once he stops you.

Anima’s Bad Ending. Although the credits roll, unlike the other options, it doesn’t have an Ending Star. Do not pull the Trigger against Yamato (You are forced to do up to joining the Rail Cannon, just don’t pull the trigger).

Unlocked after finishing any of the Anima or Noa’s endings.

As you start near the start of the 2nd Branch, during decisions (both for answers and actions, petals will surround you, if you select the right choice, they will ascend while disappearing. Do this constantly to enter the new Branch.

The new branch starts in the Star previous to Noa’s 2nd Concert, as it will be where another branch opens on the other side with the same concert. The map’s direction is from right to left. It ends near the start of the map.

  

Deepen bond with Unique Meteora Anima.
Guaranteed. Be nice to Anima in the final branch by giving her items related to Coco

Witness Noa and Anima’s resonance.
Guaranteed. Get Noa to sing with Anima

Defeat Noa in battle.
Part of the story. Just in case, I selected Julie when asked who I should investigate. I believe Dieter may have a path for him too? I couldn’t find the option again…

Defeat Altdeus in battle.
Guaranteed. Defeat Altdeus, the Final Boss.

Both are available on the same final star, based on the choice you make, hence, both are True Endings, and none is more canonical than the other.