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
No enemy ships harmed in 'Gridiron'
Shoot the saw through the two large chambers and live to tell about it in 'Football Racetrack'
Complete a World 2 Marathon Run
Die 100 times on one level
Complete a World 1 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'
No saws dislodged in 'Stomach II'
Beat Omegalith
Collect all Atomiks in World 1
Beat every level in World 1