Finish Evacuation within 18:00 without dying on Heroic difficulty.

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Just rush through the driving section, skipping as much combat as possible. When you do have to use your weapons try and use automatic weapons to remove extra armor off enemies. Then use an accurate weapon (pistol, BR, DMR) to finish off damaged enemies. When you get to the Promethean Knights try to shoot for the glowing bits on each side of their head. Once one is shot off they will yell, exposing their face for a easy headshot.

In addition, if you go down and are revived you can still get this achievement. If you die and you are quick enough you can pause and reload your last checkpoint.
All in all, you just want to run through as quickly as possible avoid combat where unnecessary.

Achievement is worth 10 Gamerscore.