Here is the full list of all 20 Downwell achievements.
Beat the boss (hard mode)
Beat the fourth area (hard mode)
Beat the third area (hard mode)
Beat the second area (hard mode)
Beat the first area (hard mode)
Beat the boss
Beat the fourth area
Have more than 5000 gems
Land a 100 combo
Have more than 3000 gems
Beat the third area
Complete a level without landing
Land a 30 combo
Complete a level without killing any enemy
Beat the second area
Beat the first area
Land a 10 combo
Complete a level without shooting
Complete a level without taking damage
Complete a level without visiting siderooms