Here is the full list of all 40 Grim Guardians: Demon Purge achievements.
Saved Risu
Cleared stage 7
Performed Sister Rescue a total of 20 times
Cleared stage 6
Defeated a total of 500 demons
Found the secret in stage 6
Rescued 10 students
Cleared stage 5
Found the secret in stage 5
Cleared stage 4
Purchased a total of 20 items at the store
Performed a character swap a total of 100 times
Defeated a total of 250 demons
Cleared stage 3
Reloaded a total of 30 times with Shinobu
Cleared stage 2
Cleared stage 1
Reached the normal ending
Reached the true ending
Reached the alternate ending
Unlocked the gymnasium
Rescued 20 students
Rescued every student
Defeated a total of 1000 demons
Performed the Demon Hunter's secret arte a total of 20 times
Defeated a demon with one of Maya's punches
Returned to the start of the game
Handed the 5 items over to Houdai
Selected "Believe in the bunny" before going into the dimensional rift
Selected "Look for another way" before going into the dimensional rift
Powered up all weapons
Cleared Boss Rush mode on Casual difficulty or above
Cleared Boss Rush mode on Veteran difficulty or above
Cleared Boss Rush mode on Legendary Hunter difficulty or above
Found the secret in stage 1
Found the secret in stage 2
Found the secret in stage 3
Found the secret in stage 4
Found the secret in stage 7
Obtained everyone's panties