Once you've finished the fight and have gotten your Legendary achievement you probably are wondering what do I do now?  Well you could watch some saved films or even play around with Forge or possibly hop online for some match making.  Those are all a possibility but you may want to check out some of the other options like playing the campaign again but this time for score. 

The Campaign Scoring meta game gives you a score for how you play the game.  You get more points for double kills and assassinations than for just mowing down a grunt with your assault riffle.  Better yet, you can play co-op campaign scoring and either play as a team or against each other.  There are achievements to be unlocked it you can reach a specific score in in the campaign.  That score is not easy so you may need some help.

In every Halo game Bungie has hidden skulls on the campaign levels.  Each skull adds some form of difficulty to the game.  As you find skulls and turn them on you will gain additional points in a level.  Some skulls are funny, like the grunt birthday party, when you kill multiple grunts there is confetti and cheers.  Other skulls make your shields power up slower or give you a health boost when you bunch a fool.  Since the skulls are hidden and often put in places you just would not bother to look, it's a task to hunt them down.  Finding certain skulls is an achievement and every level has at least one. 

So once you've completed the campaign, modifying scoring will delete you current save, go give this a try.  I can't wait to get some you chowder heads in a co-op campaign scoring match and see if you can hold your own.  I'm guessing not.  After each level there is a stat central just like in multiplayer.   Hopefully eventually this data will be online linked to your Bungie account so people can see Tim died 50 times on Assault or that I've killed 300 brutes over the campaign.