Halo xBox cheats not available. But you can use following tips and tricks:

Kill Anyone
Get any vehicle like Warthogs , drive fast and then go slightly to the side of an enemy and quickly spin when you're almost beside them. You will run them down and kill them.

Big Battles
Whenever you get into a big battle with covenant and flood jsut let them fight it out until they kill each other.

Killing elites in the fort
When you are in a warthog and on a hill about to run into one of the forts, get out, switch to sniper, zoom x8 and kill elites that you see only do this for the soldier rescue things because there is ammo everywhere for sniper rifles in that part.

More health
It work sometimes. it do it is X,A,R1,R1,L1,X,B,A

Walkthrough of Halo

Go to following link
http://www.gamefaqs.com/console/xbox/file/472132/15651