Here is the full list of all 46 Rusted Moss achievements.
Discover the full map
100% the game
Reached Ending D
Fully upgrade gun.
Get all upgrades
Construct Titania
Join the Tea Party
Read all the lore boxes
Reached Ending C
Reached Ending B
Reached Ending A
Get all trinkets
Beat every boss in a single run
Survive Eli and Nell's fight
Defeat Famine
Beat all climb challenges
Clear Juni's Climb Challenge
Return The Face
Defeat Face Stealer
Clear Fern's Climb Challenge
Defeat Spirella
Get all the guns
Defeat Maya
Get a Gift from Abby
Defeat Void Worm
Clear Shopkeeper's Climb Challenge
Defeat Freia
Defeat Pestilence
Clear Seer's Climb Challenge
Defeat Lenore
Defeat Elicia
Clear Ameli's Climb Challenge
Defeat War
Get Shot With a Huge Laser by Maya
Find your first hidden note
Defeat Ameli
Defeat Bonnie
Beat game in 2 hours
Reach the lowest area in 15 minutes.
Get first Titania piece from Maya
Beat the game without talking to the Seer
Get all achievements
Clear the game with less than 4 Titania pieces
Achieve ending A in speedrun mode under target time.
Achieve ending C in speedrun mode under target time.
Achieve ending D in speedrun mode under target time without giving the Seer any Titania Pieces.