This one is really simple if you watch the video and follow exactly what this guy does. One of the better runs I've seen, thought id share with others.
Replay the video a few times to get the run down-pat & follow his exact steps, you will surely prevail!
@ 2:09 - He grabs the Gravity Lift that you will use to jump the broken highway with the warthog.
@ 2:24 - He throws a grenade to destroy the Force Field blocking the tunnel, in order to enter with the warthog.
@ 2:58 - He uses the Gravity Lift (you picked up earlier) to jump over the gap in the highway. ***MAKE SURE THAT YOU PLACE IT AS CLOSE AS POSSIBLE TO THE LOCATION IT WAS THROWN IN THE VIDEO, WARTHOG IS A MUST!***
@ 3:13 - I recommend going a tad bit slower thru the construction pipes, as to not get stuck and lit up by enemy nades. ***LIKE I DID

***
@ 4:24 - He shoots the Force Field with the Fusion Rockets to activate the Cut-Scene, which after moving up closer (as he does in the video) will end the mission.
***BE VERY CAREFUL, AS THE PLASMA TURRET WILL LIGHT YOU THE HECK UP!!!***
Everything else should be self explanatory after watching the video very closely.
GOOD-LUCK!
Note: Speed Run on Legendary Difficulty. All Rights reserved to the makers of this video.