Description is self explanatory, simply beat this level on Legendary difficulty. The reason I am postin a solution is because this will not unlock for some people because of a glitch. The way around this glitch is instead of starting a new campaign on legendary select each level individually from the timeline and select legendary difficulty. At the completion of the level your Achievement will unlock.
edit: Park Diesel said in the comments that you can not skip cut scenes for the achievement to pop, I skipped them but if you still are having an issue this may be a solution.
edit 2: Accoring to Ibanez V7 It's nothing to do with skipping / not skipping cutscenes. As AP MisTaKeZ pointed out, the top left of the screen should read "Health Regen +0". Ibanez V7 had to go in and out of the 1st level twice before they had a 0.
As posted on x360a:
**WARNING: Although the achievements themselves don't appear to be glitched, the game seems to have trouble recognizing when you are selecting 'Legendary' difficulty for some people. To be sure you are playing on Legendary as soon as you start Chapter I check to see what your base execution bonus number is, for Legendary it should be '0'.**