Acquire the Scarab Gun
Shoot the soccer ball in the level Metropolis
Halo 2 SCARAB GUN AND SOCCER BALL – Achievement Guide
Complete Skull and Terminal CHECKLIST(Halo 1-4 / Google Docs)
Jump into the Warthog and leave your allies behind. Once you get to the tunnel, turn around and wait right outside of the tunnel on the left lane (as you are looking towards the bridge). Wait until a Banshee shoots at you and drive backwards. Go as far as you can and exit once you hit a wall or car.
Now here starts the tricky thing. You will have to lead this banshee through the whole tunnel. Every time it passes you it will turn around and fly back out. That’s what you don’t want. A good method of preventing that, is to stand still when it passes you. You can also shoot it a couple of times and it will come back.
It will probably get stuck somewhere a couple of times. Just move around it a little and it should follow you again.
This next part is very important. Lead it into the room at the end of the tunnel and turn left. This is where the new area will be loaded. Stay close to the Banshee now and wait for the exact moment when “LOADING…DONE” is displayed on the screen. Enter the Banshee now and fly through the small tube into the open area.
If you miss the right moment the Banshee will disappear, but you can simply reload the last checkpoint and try again.
That was the hard part. From here on, simply follow the video to the Scarab Gun and the Soccer Ball. The Soccer Ball has to be shot in order to pop the achievement.