Halo Reach: Complete any campaign mission on Legendary in 12 minutes?

The weekly this week is to complete any campaign mission in 12 minutes or less. What would be the best mission to do this on? I don't really play the campaign that much and I would love to show off to my boyfriend that I got the weekly done before he gets home. So what would be the best mission to do this on, and any tips as well. Thanks!!