Halo 3 Covenant Stencils In Halo 3, Can You Simply Complete The Last Mission On Legendary To Get The Legendary Achievement?

In Halo 3, can you simply complete the last mission on Legendary to get the legendary achievement? - halo 3 covenant stencils

In Halo 3, I finished 6 / 9 missions, on the legendary, but 3 / 9 Normal. It's strange how I do most of the missions, on the legendary, but normal driving. I really want the legendary and heroic deeds, but I will do everything to get the last 3 missions to the legendary success. Then I beat the last mission on legendary to get the achievement? If you can, please explain everything. Thank you.