The "Where Cubbage Fears to Tread" achievement is earned during Chapter 7, "Highway 17". Upon reaching the resistance outpost known as Little Odessa, you'll meet Colonel Odessa Cubbage, who provides you with an RPG launcher. Shortly after, a Combine gunship attacks the outpost. To earn the achievement, use the RPG to destroy the gunship.
Be aware that the gunship can shoot down your rockets. To counter this, guide your rockets in unpredictable paths using the RPG's laser sight, or wait until it finishes firing a barrage to evade its defenses.