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