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