Halo 3 Devastating Achievement: Sierra 117

Here is a speed run guide for the mission Sierra 117 (legendary difficulty)
Hello everyone and thank you for checking out my Halo 3 Speedrunning guide for the Devestating achievement (beat all Halo 3 missions on Legendary in under 3 hours). If you match the part time on every mission then you will clock in at 2 hours and 40 minutes. A lot of missions are a lot easier or harder to hit that par time, so here are my recommended times for each mission:
Sierra 117 - 20:00
Crow's Nest - 20:00
Tsavo Highway - 10:00
The Storm - 15:00
Floodgate - 10:00
The Ark - 30:00
The Covenant - 30:00
Cortana - 15:00
Halo - 25:00
Total - 2:50:00
In case you want to compete against my times, here are my personal best on each mission!
Sierra 117 - 10:06 (1 Death)
Crow's Nest - 12:19 (2 Deaths)
Tsavo Highway - 4:51 (2 Deaths)
The Storm - 10:04 (3 Deaths)
Floodgate - 4:31 (0 Deaths)
The Ark - 16:55 (4 Deaths)
The Covenant - 21:00 (9 Deaths)
Cortana - 9:41 (0 Deaths)
Halo - 13:25 (1 Death)
Total - 1:42:52

[@Matthew-up9tx]

after you free johnson, can you just sit in the back and camp till the pelican comes or do you have to kill the enemy’s?

[@TheArmyofOtters]

You can wait if you want, but it will take longer. The faster you clear the area the faster the pelican will come in, but it will come eventually.