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