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