In Halo: CE go to the level Assault on the Control Room, and play it all the way to the part where you see Elites taking off in Banshees. If you have a Sniper Rifle, try sniping them. If it doesnt work, revert to a checkpoint until you can kill one. Once you`ve done so, take his Banshee and fly down to get a Ghost. Getting the Banshee is not required but it also triggers no enemies to be spawned on the pyramid structure, which enables you to get there faster. Drive your Ghost all the way up to the top where you can open the door. But don't open it or you will have done all this for nothing. Instead drive your Ghost up to the white triangle, hover there so that you are directly over it and make sure the door is directly to your right. Now begin strafing to the right and press up against the door. By that time, you should be able to see a Grunt or two on the other side. If you can see the Ghost at least partially on the other side, you know you are doing it correctly. Now once you are completely against the door, exit the Ghost. If done correctly, you shouuld exit on the other side of the door. Once there, you will still see all the Elites and Grunts except they wont attack you. You can do some pretty funny things to them as well. To re-activate them, just enter your Ghost again, pull away from the door, and activate the control panel.
If it doesn't work the first time, don't give up, and try at different angles.