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