Here is the full list of all 20 Atomik: RunGunJumpGun achievements.
Collect All Atomiks in World 2
Beat Omegalith
Collect all Atomiks in World 1
No enemy ships destroyed in 'Target Locked'
No saws dislodged in 'Stomach II'
No mercy for turrets in 'Pew Pew Pew'
Die 100 times on one level
Complete a World 2 Marathon Run
Beat every level in World 2
Complete a World 1 Marathon Run
Beat every level in World 1
Beat every level in World 3
Collect All Atomiks in World 3
Beat Omegastroid
Beat Omegaton
Collect all Atomiks
Complete a World 3 Marathon Run
Beat a level without using the fly button.
No enemy ships harmed in 'Gridiron'
Shoot the saw through the two large chambers and live to tell about it in 'Football Racetrack'