For this achievement you must finish all the missions for Halo CE in Heroic difficulty or higher, it will stack with legendary. You can do this in SP or co-op.
You must be careful if you're using skulls as some may void the achievement. These skills have a warning on them before you start the mission (anything with a x0.00 multiplier).
If the achievement glitches you can: restart the game, restart the console or go to the game hub and tab to 'progress'. The last method appears to be a new feature that refreshes you're stuck achievements.