Jagged Alliance: Rage!

Once you've crossed the bridge and accessed area 2 you can secure the Water Reservoir near the center of the map. Part of the way through the mission, once the alert has gone up, an enemy will start poisoning the water basins. You need to kill him before he completes his task. The achievement should pop with his death.

The first time I tried this mission the achievement didn't unlock even though he only poisoned two of the four basins so I loaded an in mission save, killed him again and it worked fine.