http://guides.ign.com/guides/734817/page_2.html it is a guied to all of Halo 3 (copy and paste)
No it does not. You have to beat every level.
Depends on what level You beat it on.
get a new halo 3 disc
no no
Cortana =]
Just follow what Cortona saids.
coastal highway
If you are looking for a detailed guide, I would try YouTube for a level walkthrough.
only in the last level of halo2 and all of the game in halo 3 :)
No, you free her at the second last level.
I think it is "The Maw."
Unfortunately you cannot beat halo 3 by just beating the last level. You need to do all the levels. If you want to get the Heroic and Legendary achievements then you must do all the levels at least once on Legendary. Although it does not matter whether it is on co-op or single player. I wouls suggest doing the levels on co-op with as many players as you can because someone can run out and if they die they can just respawn at the other person rather than having to go back to the last checkpoint.