Here is the full list of all 41 Demon Skin achievements.
Start the game
Start the Ice Cave
Start the Underground
Start the Forest
Start the Castle
Finish the game
Defeat the Fire Skeleton
Smash the big golem head
Get away from the avalanche
Defeat the Yeti
Finishing off with an axe (Up Stance)
Finishing off with an axe (Middle Stance)
Finishing off with an axe (Down Stance)
Finishing off with a hammer (Up Stance)
Finishing off with a hammer (Middle Stance)
Finishing off with a hammer (Down Stance)
Finishing off with a sword (Up Stance)
Finishing off with a sword (Middle Stance)
Finishing off with a sword (Down Stance)
Fall into the lava
Fall into the spikes
Drown
Fall into a trap
Get killed by a fan
Get killed by acid
Die under the pressure
Get killed by gas
Die in the fire
Find the first Crystal
Find the second Crystal
Find the third crystal
Find the fourth Crystal
Moon blade Super Strike
Forest axe Super Strike
Blue hammer Super Strike
Fire hammer Super Strike
Fire sword Super Strike
Bloody axe Super Strike
Kill the ghoul yourself
Kill the first necromancer
Reach Level 12