Here is the full list of all 31 Crystal Guardians achievements.
Defeat Gilbert in wave 20
Defeat the zombie horde in wave 10
Defeat the horse raid in wave 30
Defeat the king skeleton summoner in wave 40
Defeat Gilbert in wave 20 in hardmode
Defeat the zombie horde in wave 10 in hardmode
Defeat the Horse raid in wave 30 in hardmode
Defeat the king skeleton summoner in wave 40 in hardmode
Kill a total of 1000 enemies
Kill a total of 10000 enemies
Kill a total of 50000 enemies
Earn a total of 10000 gold
Earn a total of 100000 gold
Earn a total of 250000 gold
Complete a run while only buying fire crystals
Complete a run while only buying projectile towers
Receive a total of 100 random items
Receive a total of 1000 random items
Receive a total of 5000 random items
Defeat the Crab King in wave 70 in hardmode
Defeat the Greatest Grandmother Slime in wave 50
Defeat the Greatest Grandmother Slime in wave 50 in hardmode
Defeat The Wizard Club in wave 60
Defeat The Wizard Club in wave 60 in hardmode
Defeat Gilbert's twin in wave 80 in hardmode
Defeat the Goblin invasion in wave 90 in hardmode
Defeat the Corrupt Angel in wave 100 in hardmode
Defeat normal mode (max wave 40)
Defeat normal two-lane mode (max wave 60)
Defeat hard mode (max wave 80)
Defeat hard two-lane mode (max wave 100)