Par time for this mission is 15 minutes. In order to achieve this low time and make it a whole lot easier on yourself, you'll have to take advantage of the "shield bump." The shield you will use is right at the beginning of the level, so it will make beating legendary on Keyes much easier. You will completely skip the part where you go out of the ship and have to fight back to the gravity lift. You will go directly to where
Make sure to turn on the time hud so you know where you are with respect to time. Also, remember that while you will skip a majority of the mission, you still have to complete the rest of the level from the
I recommend doing this achievement on Easy so the rest of the level will be rather, well, easy. I performed the trick with about 6-7 minutes on my time hud and competed the level on Normal at about 10 minutes. So, on Easy it shouldn't take no more than 3 minutes; therefore, restart the level if you can't pull off the trick before the time hud reaches about 11 minutes.
Below is a video I used and believe is the most helpful. While he is demonstrating the trick on PC, it works exactly the same on the Xbox. I have also given the link to the reference position you will want to use depending on your resolution. This is also on PC, but it is the same once again. I was a little
bit to the left from the given screenshot however.
A few tips I can give you is to use the Plasma Pistol when lining up your view. The top crosshair thing really helps. You want to have that top one going through the corner of the shield base (by using a straight imaginary line). You want to be looking directly downward and have the middle of the crosshair triangle to be on that horizontal line. You need the shield to be vertical on your screen and have none of the "jaggedness," as stated in the video. You can also periodically shoot the shield generator so it won't respawn and give you more time to line up your screen. Crouch when lining up your view to make it easier to see your position!
After achieving the shield bump and reaching the room, you will need to keep going through the door in front of you and take a right, past the fire, and trigger a checkpoint. Now go back to the room; you may see flood and covenant floating still in the air, don't worry about them or kill them. Once back in the room, there will be invisible flood (you can tell because your reticle will be attracted to them). You need to kill these invisible guys, either with grenades or enough bullets, or else they will spawn during the cutscene and kill you. This will cause some weird glitches to occur (at least it did for me the first time).
Lastly, you want to be in classic graphics as this will make it much easier!
This trick is very precise, with only a few pixels separating success and failure, but it is doable!*Credit to screenshots go to goatropehttp://imgur.com/a/UmAmT#12*Credit to YouTube video goes to Andrew Halabourda