Here is the full list of all 22 Hellmut: The Badass from Hell achievements.
Enter dungeons as the Rat King.
Die for the first time.
Shoot 10,000 shots.
Unlock all Transformations.
Buy one of each item type in shop.
Kill one of every enemy type.
Stay in the shop for more than 20 minutes continuously.
Don't lose a single hitpoint in one level.
Complete Challenge mode in under 1 minute.
Shoot 1,000 enemies with a single weapon.
Complete one playthrough in under 20 minutes.
Defeat The Demon.
Acquire Professor Transformation.
Enter dungeons as the Orc Fairy.
Collect 5,000 Soulstones cumulative.
Kill at least three enemies with an exploding barrel.
Complete Tutorial.
Use every weapon.
Kill The Demon at least 10 times after receiving Professor Transformation.
Play with all Transformations except the Professor.
Deal over 100,000 cumulative damage.
Do 15.000 damage with barrels.