Halo Cheats

This will help if your annoyed by always having to go back because the yellow dude locks the door. First find a warthhog and get some people inside. Go where the place where there are lots of guys and where kortana says the covenant are putting up a real fight. Go down completely with your warthhog and go at full speed directly where the yellow dude is. If you did it right then your warthhog will be half way through. Press forward and get out. Then pass the level like you usually do and come back where the warthog is stuck. You may think it is impossible to get out but just press back and then get out. You will be in back of it and now you're done the hard part!

While driving in a Ghost, hold A. This is fun when you drive off rocks you'll go flying!!!

Get a friend. Have your friend sacrifice himself until the room is clear.
Always use a good combination of weapons, and think about your surroundings and enemies when choosing weapons. The sniper rifle is great until you reach the Flood. A good combination is the shotgun (the human pistol before you reach the Flood), an automatic weapon, and grenades. If you do not prefer to follow this example, then remember to use a combination of short and long range weapons.
Conserve ammunition. Never waste ammunition, always gather as much ammunition as possible, reload your weapon during any break in the action, and switch off any plasma weapon for a better one whenever possible.
Take it slow. Do not burst in to take all of the fire. Also, use the scenery as a cover. If you can fit behind it, you can hide behind it.
Use the enemies themselves to your advantage. Remember that each enemy type has their own mannerisms and characteristics.
Take shortcuts when possible. Here are some examples. In The Silent Cartographer level head directly to the security override station. In the Assault On The Control Room level, after you get one of the early Banshees, in the area after you go under the tunnel you cannot take the tank through, the double bridge will be directly above you. Ride the Banshee to the bridge and skip a large portion of that level. Taking this shortcut means there will be no more enemies throughout the rest of the level. In the 343 Guilty Spark level, go to the secret area with the 2-D trees and follow the cliff through the structure that goes indoors. In The Maw level, after the dropship gets taken down, you will keep going. Continue until you see the first open door on your right. Enter there and take another right. This can shave a good minute and a half off the time it takes to get to the Longsword at the end of the level.

Here's a good way of killing elites or hunters in legendary. get a
needler and some ammo and just unload it on an elite and watch the
show --NOTE-- You have to get in close enough for the needles to find
their mark, and the delay until they explode leaves you wide open for
their plasma rifle, so bring a backup weapon.

For the hunters, unload an entire clip at their backs so the shot doesn't
reflect off their shield and start shooting at them with another
weapon while waiting for the needles to explode (or just sneak up on
one and smack him in the back, and if you cant to that...a well placed
plasma grenade to the head should weaken it, but it will most likely
just piss it off.)

In the first leval when you get a gun when the people explode from that grenade the left person that was garding was jonson