Wasteland 3

After the final boss battle in the room with the Holy Detonation, Theo will talk to you about approaching the computer terminal to decide the fate of the Cheyenne Mountain. Make a save here before interacting with the computer if you're trying to get both ending achievements.

While on the computer you will have three options pop up on the screen. You must click the one that says "Reverse time dilation". This will shut down the Holy Detonation and kill a few nearby viewers. The achievement should pop when this happens.