Here is the full list of all 12 Zenith achievements worth 1,000 gamerscore. It takes around 8-10 hours to unlock all of the achievements on Xbox One.
Finish the game
Defeat 500 enemies
We need to protect the pure!
Find Brekka
Defeat the lava demon.
Destroy 100 barrels
Save the Skree
Destroy 50 barrels
Defeat 50 enemies
Recover the artefact
Defeat 25 enemies