From Halopedia, the Halo wiki
This is the final level of Halo: Reach, in which Noble Six dies at the end, no matter how skilled the player is. The objective is therefore to survive for as long as possible. With the right combination of weapons and equipment, it might just be possible to survive indefinitely...
There'll be Another Time... (Rally Point Omega)
The objective is simply to survive for as long as possible. Here are some tips:
There's an exploit allows you to survive indefinitely without doing anything. Just crouch in the corner of the Armor Lock room, behind the three crates. If the crates are positioned correctly, Elites will not see you.