You first need to unlock Captain Naughty. Complete levels 3-1, 3-2, 3-3 with at least Bronze trophies.
I used episode 3 story level (3-0).
Work your way through the level til you reach the final section. You will see Cozy with the blue helmet. Colonel Ketchup is the military fellow with sunglasses and a fancy hat. To start with, no one cares about you. This is a great time to pop Cozy with the tv if you need the achievement. Once you cause enough issues everyone will be onto you and you'll need to be more careful.
You need to get Colonel Ketchup injured enough that you can get behind him to perform the oozy kill. You can use traps, scare him, beat him, etc. I decided to sit in the bushes and take pop shots with the oozy at him until he started limping.
Quick notes:
- Make sure you keep the oozy until you pop the Colonel
- Don't let the Colonel become scared stupid (Bubbles over their head)
- Don't kill the Colonel by accident
Good luck and have fun