Important notes for getting this achievement after several annoying playthroughs.
Firstly, medpacks do not count, if you hurt yourself and heal back to 100% your achievement will not unlock.
Secondly, if you hurt yourself and reset to last save or restart mission, it still will not count.
Basically, if you hurt your health in any way, you will need to quit and restart the mission otherwise the achievement will not unlock.
All these were discovered painfully after nearly 8 playthroughs of Pillar of Autumn on Heroic
A tip though. Every few checkpoints, save & quit then reload. Now if you die, go back to your dashboard, reload the game and resume. All is forgiven and you're back where you were at the last time you saved & quit. You may need to run through one or two more checkpoints again, but it's easier then repeating the whole level.
PS: thinking about it, the number of times Halo games have had this exploit, you' think they'd have solved it by Reach now, huh?