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