This is a story achievement and you can't miss it. After this person talks to Marianne:
You're chased by the creature you're supposed to defeat, leading to a generator. Then follow the linear path, picking up the old gas can and lever (there are a few balance obsticals along the way).
When you return to the generator, insert the lever on the wall, then interact with the side of the generator. Look at the top, remove the cap, and fill the generator with the gas. Now look to the right and turn the generator on. Finally, exit the generator and walk in front of it. Now, press
and push it into the water to electrocute the creature. The achievement should pop when the generator touches the water.