Here is the full list of all 33 ONI : Road to be the Mightiest Oni achievements.
Perform a 20 Spirit Slayer combo.
Perform a 35 Spirit Slayer combo.
Extract spirits with Kazemaru 100 times.
Extract spirits with Kazemaru 500 times.
Defeat 500 enemies.
Defeat 1,000 enemies.
Capture 3 rare demons.
Capture 9 rare demons.
Save 12 spirits.
Find 2 cursed talismans in the forest.
Collect 5 rare mushrooms on Kisejima.
Collect all demon pants on Kisejima.
Find Princess Nayuta's grave in the cemetery.
Save all spirits on Kisejima.
Collect all weapons on Kisejima.
Open all treasure chests on Kisejima.
View the scene entitled Meet Kazemaru.
View the scene entitled Reunite with Zenisuke.
View the scene entitled Meet Baby Boar.
View the scene entitled Kuuta and the Girl.
View the scene entitled What's Your Name?
View the scene entitled Peaches and Tofu.
View the scene entitled Flowers.
View the scene entitled Let's Play!
View the scene entitled Training Continues.
View the scene entitled A Promise.
View the scene entitled To the Final Battle.
Defeat Guardian Dog #1.
Defeat the legendary demon, Bunta.
Defeat Guardian Dog #2.
Defeat the legendary demon, Rikiya.
Defeat the hydra guardian, Orochi.
Defeat the legendary demon, Jiso.