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