I have a rough idea from what i have heard my little brother and his freinds do.
You do not need to "mod", but you do need to "standby". Mod and you will get caught! You will need to set up your xbox live through your computer and have it also be protected by your computer's firewall (i believed he used zone alarm) , then you will need to set up a "allowed" ip address(s); so when you have your firewall turned on, no ips can have connection but you. What this will do if done for to long (more than 2 seconds) is; everybody will timeout and the match will be over. So, you need to get a macro that switchs between firewall settings (off/med/high) every 2 seconds, so your opponents will lag on their screens and allowing you to shoot their roaming characters. I would verify all info seen above on halo forums, or cheat websites, but i think this will point you in the right direction.