He's Unstoppable Achievement in Halo: The Master Chief Collection

The Pillar of Autumn is without a doubt one of the best missions to attempt this achievement given the fact that it’s the shortest mission within the entire Campaign. You’ll also come to realize that given the nature of the Autumn’s design structure, it’s also the most confined mission in terms of navigable space for firefights. This provides for an excellent tactic in flushing out rooms with grenades if the Bandana, Boom, or Grunt Funeral skulls are activated.

As such, make sure to utilize grenades as often as possible when taking on groups of Covenant enemies inside the Autumn in order to avoid direct confrontations and thus taking health damage. Since the corridors here are tight, a single grenade can usually flush out an entire room or hallway of enemies with ease. Just make sure to stay back when tossing the grenades if Boom or Grunt Funeral have been activated.

An alternative strategy for this achievement is to tackle the mission cooperatively with another player. This method is great since your partner can go forth and clear out all the enemies, while you stay behind and avoid any and all crossfire between your partner and the Covenant. If you are successful in completing the mission without taking any health damage whatsoever, the achievement will unlock after the concluding cutscene.