You just need to finish the fight
What really makes this game is the mutliplayer. If you have played halo 2 online before it feels about the same thing just with more weapons and equipment. If you haven't played halo 2 online then this is an experience that you shouldn't miss. Halo's multiplayer really stands out because it's just really good. The game was really meant for playing the multiplayer portion. The maps are really cool, the weapons are weighted pretty well, and the vehicles and a great balance when you are playing with 15 other players.
Halo 3 is an awesome game, but if you don't plan on playing the multiplayer side of it over xbox live or playing co op just rent the game and beat it's single player.