Instructions
Choose the heaviest weapon you can lay your hands on. You'll need serious firepower to penetrate the Phantom's armor. The best choices are the Spartan laser or rocket launcher. If you have access to a Warthog, Wraith or Scorpion, their heavy weapons are even better.
Disable the Phantom's weapons. The Phantom mounts three plasma cannons, two on its sides and one under its nose. Once you've destroyed those, you can concentrate on your target without having to worry about return fire.
Target the gravity lift on the Phantom's underside and the entrances to the troop bay on the sides. Covenant forces deploy from these and you can rack up a few kills by targeting them as they exit the Phantom.
Concentrate your fire on the Phantom's tail. This is where its armor is weakest. In "Halo: Reach," however, the Phantom's armor appears to be equally strong all across its body.