Here is the full list of all 16 Crystal Guardians Demo achievements.
Kill a total of 50000 enemies
Defeat the horse raid in wave 30
Earn a total of 250000 gold
Earn a total of 100000 gold
Kill a total of 10000 enemies
Defeat Gilbert in wave 20
Defeat the zombie horde in wave 10
Kill a total of 1000 enemies
Earn a total of 10000 gold
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
Complete a run while only buying fire crystals
Complete a run while only buying projectile towers