At the start of Chapter 6: Collateral Damage, you will be driving a tank. It is a relatively lengthy section that lasts for approximately ten minutes. While driving the tank, there will be enemies with rocket launchers and enemy tanks shooting at you. The armor of your tank must stay above 80% to get the achievement at the end of this section. Your armor is shown in the bottom left corner. However, it does not show the percentage you have remaining -- so you will have to guess. Try not to get hit in the beginning at all. Stay as far back as possible and shoot at the buildings to clear out enemies. Later on there will be more difficult sections. At one point, there will be two tanks driving around the map. You have to drive behind buildings so they lose sight of you, then ambush them from behind. Two shots from behind (or one in the back, one in the side) are enough to destroy an enemy tank. Just avoid shooting them at the front, as they are heavily armored in that area and will shoot back. Also, set the difficulty to "Rookie" to make this easier. Restart from the checkpoint if you get too badly damaged.