This is in Act 4, Chapter 1 right after Ishi threatens Trishka when she tries to escape. You'll go to the bottom of the street and there's a barricade blocking your way, and Trishka says she will give you 15 seconds to figure out how to get past the barricade. At this point go up the street and look at the top of the buildings on the right hand side to see a large globe. Shoot it down and it will clear the way, and this will cause the achievement to pop a few seconds later. Credit for the video goes to jhaller2