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