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