Here is the full list of all 21 Void Invaders achievements.
Complete Hardcore Mode
Complete a level on less than 10% health
Score over 250,000 in Wave Mode
Score over 250,000 in Endless Mode
Fire 1,000,000 bullets
Complete Level Mode
Fire 500,000 bullets
Destroy 2500 enemy bullets
Grab a power-up after completing a level
Complete a level without losing health
Pick up 100 powerups
Complete a level without moving vertically
Kill 2,000 enemies
Kill 1,000 enemies
Die 50 times
Stand still for one minute
Reset the game
Unlock your first skin
Finish the tutorial
Play the game with the volume turned down
Die in the tutorial