I've tried every route possible and still can't avoid getting blown up. I read that note the guy gave me but am not seeing those building I'm supposed to take cover under either. I've even tried telling my followers to wait and doing stuff that cranks my agility up to 10 but still can't make it to any building that looks like cover. None of the guides here have been completed enough to have any of the quest that involve this section.
I've also tried taking the long route and finding that subway tunnel but can't take that yet because it requires lockpick 100. (I'm also running into a glitch where a computer screen pops up occasionally when trying to pick a lock but that's another story)
When you come into the field that they fire on, hug the left most rock wall, and go forward that way. You will take damage, and possibly lose your right arm/leg, but you'll be alive when you hit the fence, and when you do, they'll stop firing.
there's a mountain with the obnoxious invisible wall but you can climb the edge & circumvent part of the field. Basically go up top, go towards Nellis and drop down & keep going left. You can get through without a single hit if you time it right.