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.
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.
There is no way to beat the last level you can keep going on but the only outcome you'll get is the same ending and death every time, all you could achieve on this is how long you can survive and that's it. This level is truly undefeatable.
It is just a level that you can move around in.
The last level of Halo 2 is the Great Journey and you do it as the Arbiter
Just beat it.
no no
You just beat the thingamahjig and whabam! Congratulations assembly is complete!
get a new halo 3 disc
Cortana =]
The last level is 999 The last level is 999
You beat every level until you beat the last level.