This is unlocked for beating the 17th mission. The level is pretty straight forward. However, on Hard or Realistic one part can be very tricky and frustrating.
Near the beginning after clearing out the small outpost Valerie insist on driving and you are forced to use the Jeep's mounted gun. The bumpy terrain and her awful driving makes it almost impossible to aim, this is where the alternative fire mode comes in handy.
Once in the Jeep press DOWN
on the D-PAD
to get 12 ROCKETS!!
They are a one shot kill on enemy jeeps and the helicopter. I didn't know the jeeps had rockets until my 2nd play through on Hard. The rockets are pretty much essential on hard, as you take A LOT of damage trying to shoot the machine gun.
The achievement will unlock once you and Val reach a big white gate. If she didn't die you will also unlock Hot Zone Date. Refer to my solution for that achievement for more help.
Hope the tip about rockets helps!