Blow up the gas tank | Act 1 - Communication Breakdown Sniper: GW2 Guide

You will reach rail tracks and should notice a walking soldier [40m]. Eliminate him and head along the rails to reach two more enemies [48m]. Take care of one of them - Diaz will eliminate the other.

After reaching the fork in the road, choose the path going up, as there's a bunch of enemies just a bit further. At the vantage point, move to the edge and shoot the truck's gas tank [70m]. The explosion will kill everyone around.