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)
Have more than 5000 gems
Beat the second area (hard mode)
Beat the first area (hard mode)
Beat the boss
Complete a level without landing
Complete a level without killing any enemy
Have more than 3000 gems
Land a 100 combo
Beat the fourth area
Beat the third area
Beat the second area
Land a 30 combo
Land a 10 combo
Complete a level without shooting
Complete a level without taking damage
Beat the first area
Complete a level without visiting siderooms