Here is the full list of all 40 Grim Guardians: Demon Purge achievements.
Reached the alternate ending
Rescued every student
Selected "Believe in the bunny" before going into the dimensional rift
Reached the true ending
Selected "Look for another way" before going into the dimensional rift
Reached the normal ending
Handed the 5 items over to Houdai
Defeated a total of 1000 demons
Rescued 20 students
Performed Sister Rescue a total of 20 times
Found the secret in stage 2
Found the secret in stage 3
Unlocked the gymnasium
Rescued 10 students
Found the secret in stage 1
Defeated a total of 500 demons
Saved Risu
Cleared stage 7
Cleared stage 6
Found the secret in stage 6
Cleared stage 5
Purchased a total of 20 items at the store
Performed a character swap a total of 100 times
Found the secret in stage 5
Cleared stage 4
Defeated a total of 250 demons
Reloaded a total of 30 times with Shinobu
Cleared stage 3
Cleared stage 2
Cleared stage 1
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
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 4
Found the secret in stage 7
Obtained everyone's panties