I Need a Hero - 40G (Original Achievement)
To get this Achievement, you must complete the entire Campaign on Heroic difficulty or higher.
I suggest you play the game on Legendary Co-op and get this Achievement at the same time as the "Wake Up, John", "The Legend of 117", "Bropocalypse", and "Bromeggeddon" Achievements.
As said I suggest getting some of your friends together or making a boosting session on here and going for it.
I always use my Hide and Seek method for completing the new Halo games on Legendary. The trick is to have 1 player stay back a bit and be the spawn point, while the other 3 rush in and do all the killing. This way when one of the "Seekers" dies from the enemy, they will respawn on the "Hider". Once you clear the area, the Hider can move up little by little and close the distance.
Doing this will help quite a bit and shorten the time it will take to complete the game, although it will still take probably 5+ hours.