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.
Defeat 25 enemies
Defeat 50 enemies
Defeat 500 enemies
Destroy 50 barrels
Destroy 100 barrels
Recover the artefact
Save the Skree
Defeat the lava demon.
Find Brekka
We need to protect the pure!
Finish the game
Defeat 100 enemies