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