That's sofa king we todd ed dude.
Turn on the sound, whenever you hear gunshots or see Ryder say something, get out of the forklift and target the soldiers, fire with your 9mm or whatever gun you have, and get back in. BTW the number of boxes available to you on the mission is well over 6, I think four in the building and two on either side on the outside of it.
How many times have you watched that episode of ATHF?
The way I play, I don't even need sound. Like I said, get out of the forklift and protect Ryder every two boxes you load into the truck. It's foolproof. Oh, and for the second part of the mission, drive like hell and try to flip those Patriots. You figure out why once you get there.