I'll Be Taking That!
From Halopedia, the Halo wiki
I'll Be Taking That! is an achievement in Halo: Combat Evolved Anniversary that is awarded for piloting a Banshee on the campaign level Assault on the Control Room.[1] It is represented by a side profile of a Banshee in the middle of its own contrails, emblazoned on a gold medal.
The Achievement is worth 25 Gamerpoints.
Method
Just as one exits to the final canyon where Installation 04's Control Room is located, eliminate the Elite pilot guarding the Banshee, then rush to the aircraft to avoid infantry fire. Watch out for the second Elite, who would be piloting another Banshee by the time the player has taken off.
A more difficult method is to take the rocket launcher from the first large valley where you first reunite with the marines and you run into the first Wraith. (Also taking the Sniper Rifle helps with aiming.) Then proceed to the next large valley with the second and third Wraiths. High up on the cliffside is a platform with a Banshee (which will take flight and attack you if you approach the tunnel on the other end of the valley). A properly aimed rocket will knock the Banshee down from the platform.
Sources