Speed Reader 25
Complete the level “The Library” on Legendary in under 30 minutes
For most people, this is the equivalent of Reach’s Solo Legendary achievement. This alone puts the list difficulty rating up for inexperienced players.) If you’re lucky, you’ll get this in just a regular playthrough. If you’re familiar with Legendary in the Other Halo titles, you might be able to pull this off in 20 minutes or so. Again, always have a shotgun. I also recommend having the rocket launcher whenever possible. It’s good when they start to pile up. Your best bet is to find a good corner to put your back against and just keep your finger on the trigger. Bulk doors will slow you down automatically. If you don’t have a bulk door, you better be moving forward.
These video's will help alot:
They really helped me as i have never been good at this level.
Also, for all of the level specific achievement's it's alot easier to do co op with a dummy account and play that way, the main profile (the one your getting the achievement for) wont move and will be a great respawn point if you die.