Here is the full list of all 39 Swords Souls: Neverseen achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Complete endless stage 40
Beat the final boss using only beginner armor and weapons
Catch 8 different types of fish
Get 3 perfect in a row while fishing
Juggle with the same coin 10 times
Complete endless stage 20
Get three Combo medals of rank Soul Master
Defeat chapter 6 boss
Defeat chapter 5 boss
Defeat chapter 4 boss
Become Soul Master in all training areas
Catch 3 different types of fish
Defeat the Legendary Hunters
Get three Combo medals of rank Platinum
Get three Combo medals of rank Gold
Defeat chapter 3 boss
Defeat chapter 2 boss
Get three Combo medals of rank Silver
Defeat the Mama Mole
Defeat chapter 1 boss
Get three Combo medals of rank Bronze
Get three Combo medals of rank Wood
Juggle with the same coin 20 times
Juggle with the same coin 30 times
Catch 11 different types of fish
Become friends with a mercenary
Become friends with two mercenaries
Become friends with all mercenaries
Tame 3 pets
Tame 8 pets
Tame all pets
Complete the museum to 100%
Find all lost children
Trigger the Oof ring once
Get all Soul Master medals in all training areas
Complete endless stage 60
Complete endless stage 80
Complete endless stage 100
Complete endless stage 120