After capturing the Banshee on the natural bridge. Make sure to take out the Covenant tank. On the ground (relative to the middle of the land bridge) there is a crevice. Inside the crevice there is a health pack, a rocket launcher (with ammo), a sniper rifle, and regular ammo. Be careful though. Throw a grenade in first to take care of the Elite that is waiting for you with the Ghost.
After nabbing the rocket launcher, take a ghost to the second level of the tower facing the blast door. Exit your ghost from the level 2 post, run down (or jump) and open the door. Then get you ass out of there and back to your ghost, where you can take a sniper post.