During Chapter 11, you'll receive a side-quest to take a picture of your first T-47. For the achievement, you'll want to destroy it, whether before or (preferably) after completing the side-quest. This isn't easy late-game, when you actually have the tools to fight one, so fighting one this early in the game is going to be a challenge, especially on higher difficulties. To make things easier on yourself, stock up on the highest-tier explosives you can craft, preferably cannister grenades or sticky bombs, and capture the nearby Skynet outpost for the added firepower of its turrets. The fight is long, but if you can do enough damage, you will send it into a temporary shutdown, giving you time to place some shots into it's weak point before it powers up again. Once the T-47 goes nuclear, the achievement is yours.