Here is the full list of all 42 Monolith achievements.
Take a screenshot containing 100 bullets.
Won the game on Intense Lethality.
Reach a highscore of 20 minigames in the Hub.
Become the reigning tag champion.
Get an S Rank in Damage (10 hits or fewer)
Get all 4 S Ranks in one run.
Get an S Rank in Exploration (Visit all rooms)
Get an S Rank in Secrets (Find all secrets)
Finished a run with a score of 100,000.
Reach the Temple.
Protect Blastcore's smile.
This is what you wanted, isn't it?
Leave that poor cat alone.
Tarnish notte the majesty! (Acquired all Hats)
Pass the seal. (Floor 6)
Get an S Rank in Time (Under 20 minutes).
Defeated the Overlord.
Reach the Sanctum (Floor 5).
Reach the Bellows (Floor 4).
Reach the Maintenance System (Floor 3).
Reach the Archives (Floor 2).
Reach the Excavation (Floor 1).
Won without getting any upgrades.
Won without getting any weapons.
Finished a run with a score of 200,000.
Won the game on Sudden Death Lethality
Admire a fine portrait.
Finished a run with a score of 300,000.