Here it goes. The trick is how to get on top of the level "Keyes" (level 9).
You must play on co-op with a friend. Now, play thgouh the level until you get to the outside area of the level. Then, begin collecting plasma grenades (you must have at LEAST 3for this trick to work). When you have four plamsa grenades, continue through the level until you come to the second last outside area (the area just before the area when you warp back up to the ship). Now, in that area, you there should be a medium sized pool of water with a large rock in it, at the side of the mountain. It a tall rock. It's in plain sight, and isn't hard to find. Above the rock you should see a small ledge where some flood will be shooting at you. Dispatch the flood first. When you find it, have one person jump onto each side of the rock, and get as high up on the rock as you can, eithout falling off. Now, using the crouching technique, have one player jump on the others head, then jump again, so that you can uncrouch and get the other player as close to the ledge as possible. Now, here's where th plasmas come into play. Have the person on top look down and throw a grenade right on the other persons head, and have the peron on top jump towards the ledge precisely when the grenade explodes, which will propel him onto the ledge. This may take a few tries. Now, wait for the other player to warp up to you (he should be dead) and you'll notice that your still not high enough to get on the cliff. Therefore, look to the left of the ledge you are on, and you should see a second ledge that is even higher than the ledge you are on. Have one person get as close to the second ledge as possible without falling off the first ledge, than have tho other player jump on his head. Use the same grenade techiniqe, and propel yourself to that ledge. now, once you make it up (It can take quite a few tries), have one player jump down off the ledge and go into the next area, triggering a checkpoint (the checkpoint does NOT make the player on the ldge warp to you, thank goodness) so that you will not have to go through getting yourself up again. Now, then have the player on the ground kill himself, and he will then warp back up to the second ledge where the other player is. Now, once you're both up and ready, have one person stand very close to the edge of the ledge and have one person jump on top of him (again) and have him throw a plasme grenade directly on top of his head (again). Jump precisely when the grenade explodes (it takes about 2.5 or 3 seconds for the plasma to explode) and you should be propelled up and on top of the level. There is ground on top of the level, and you get so close to the Truth and Reconciliation ship that you can melee attack it :). There are many aniping spots up there, and great fews too :). You can actually walk off of the level and be walking on air for quite a while, until the level actually becomes quite small :). It's pretty neat. I hope I explained it ok. If I didn't, just say and I will explain it in more detail. Unfortunately I don't have a camera or capture cards so I cannot take any pictures of the things I do in Halo :(. So I was wondering if someone could possibly supply the pictures. If you do I'm sure you'll get a mention for the pics. Thanks.

P.S. It's a good idea to kill the person whom has the plasma grenades once after you both get up onto the ledges, because he will need to have full health to survive the grenade jump. Just have someone melee attack him in the back, and have him regenerate on the ledge and pick up his grenades. He will have full health :).