To get infinite grenades, scorpion, and ghost in multiplayer- Go to the multiplayer screen: go to edit game types: In that menu you will have the option to make up your own game type.

When one of your computer controlled buddies is in a vehicle spot/seat you want simply go over to that spot and press X. They will graciously move.

To unlock two new guns, first beat the game on the easiest setting (Easy), and then on the hardest setting (Legendary) in under 3 hours.

At the Halo start up screen, choose multiplayer mode. Choose cooperative play at the next screen, then choose your profiles at the next. Choose the level where in the beginning it says, "I would have been your daddy" (Assault On The Control Room). On the difficulty screen, choose legendary. When the level starts after the introduction sequence, have one of the players quickly press X when the "Press X to get on the Pelican" message appears. That player will re-board the ship and start flying with it down the hole. The other player will quickly die because of the aliens, and will be respawned on the spinning ring. Make sure to jump where there are gaps. If you are good enough, you can even jump to a nearby ledge. Additionally, once you are respawned on the ring, quickly jump off and fall to your death. Sometimes you will be respawned on the lowest ledge in all the blue haze, and a "Press X to flip the Pelican" will appear that the top of the screen very briefly.

When Covenant dropship is getting ready to unload a group of Covenants, throw a grenade into the door as it opens. The blast will kill several of the enemies since they are all in one area.

Use a photon weapon to destroy a Jackel's shield quickly. When you encounter a Jackel, run up and hit it with your weapon or get out a gun that uses bullets, not energy.

In the later levels of the game you will encounter new enemies called Flugs. These enemies will be small at first, but they will attack and control anything alive. Use a shotgun on these baby Flug as they tend to group in large numbers. By using the shotgun it will send a storm of bullets and kill most of them that are around. Use a shotgun on the possessed Flug, and it will kill most of them in one shot. This is made easier by the fact that most of them carry shotguns.

Successfully complete the last level of the game in campaign mode under the legendary difficulty setting. The new ending features a human and alien fighting hand to hand. The human will say "Its over", and then they hug because they know the explosion will kill them both.