During Chapter 9, you'll receive two objectives - the first is the Evacuation Protocol side-quest, tasking you with finding an abandoned Resistance bunker in the area. The second comes after you've found SkyNet's excavator, and is an optional objective on your main quest. The achievement requires you to complete that optional objective. Follow Evacuation Protocol up until you've got the C-4 (right at the entrance of the bunker). After this point, the side-quest is unimportant to the achievement, so either finish up or head back topside and make your way to the excavator. Destroy the T-800s and destroy/hack the turrets, then move in and place the C-4. After placing the C-4, you'll be tasked with finding a computer. Hack the computer to detonate the C-4 and bring the excavator down.