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