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