Hijack a Banshee during the Reach Campaign. 25 points.
The best opportunity to unlock this achievement comes near the end of Tip of the Spear. When the large Covenant spire inside the shield comes into view, there should be a Banshee flying low near the ridge you're standing on.
Pick up a Plasma Pistol from one of the Grunts you killed in the previous area and fire a charged shot at the low-flying Banshee. If you're successful and the Banshee falls to the ground, immediately move in and hold X to hijack it. If the Banshee lands far from you, charge another Plasma Pistol shot while moving toward the downed Banshee and hit it again before it's able to take off. This video is from our unofficial game guide. You can view the full guide here