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