The hardest part of this guide is going to be not choosing waterboy in the 4th episode since you're going to need the poor fella at their weakest. This part is easy enough.

The meanest part of this guide and also the easiest to do accidentally is to have him be hurt by the third dispatching section of episode 8. This will have all of his stats be at one which is what we need for this strategy. Important that this be when he's hurt as this will be the only opportunity to do so for the next step.

Now you have a hurt Waterboy with all of their stats at one by the third dispatching section of episode 8, and this is where your stats come in. You send him on the first possible mission [excluding fighting coop or sonar] which for me was atomic automatons. I'm pretty sure that's the mission everyone starts with but, can't confirm. Either way first non-coop/sonar fighting mission and your success rating will be well under 15%. Now one of two things will happen: you'll get insanely lucky and hit the 4% chance of success, which, hats off to you. Or you'll get saved by blonde blazer and succeed the mission anyway. And both will net you the achievement.

Assuming this doesn't get patched out you managed to make your own luck and get an achievement exclusive to people who can phenagle their stats and saves to get a 15% chance or less on a mission. It's that simple!