At one point in Township you will find yourself in a town square (surprise) and there are two RPF troops pinned down on all sides by gunfire from your enemies.
All you have to do is kill all the enemies before they kill either of the RPF Troops. The enemies are mainly located on ground level at each corner of the town square, however there are a few in buildings on the second level. As soon as the last enemy is dead the achievement should pop.