The first thing you need to do is play this on "easy", because the enemy AI is so overpowered, you'll need the easy advantage to keep Val alive.
Choose the Level "Swamp" (#17) or rush through the game on easy till you get there.
From the moment the level starts with you and Val hiding in front of the small camp
till she drives the nuke to the elevator, she needs to be kept alive!
And of course you can't die too, no checkpoint reload allowed!
You need to do this in one sitting.
Pretty straight forward, kill all enemies in the base,
then get in the vehicle and as soon as you drive shoot at the hangar door that opens
to get rid of the enemy car. (Use the grenade launcher)
don't bother about the helicopter, to annoying to shoot down, takes ages with the machine gun, and with grenades you'll need perfect aim.
Just kill all cars who tail you and survive till the car scene is over.
With full health and armor very easy.
When arrived at the base, go right, evade the enemies to the left side (Watch Tower, Machine gun)
there is only one enemy to kill and enter the building.
One enemy in the WC and three in the guard room.
Then go upstairs to the computer Val is talking about, let her do her thing and follow her down.
After she opens the "open sesame" door, kill both guards in front of you and the two when near the door to the second computer room.
Now the important part, when she is on the computer, and you go to collect the card and the nuke, go back through the door you came through, and kil all remaining enemies.
Because they're on their way to kill her and they will if you don't stop them.
After that, go collect the two things needed, just a few enemies to kill and follow her to the gate.
After she opens it, swtich to night vision and binoculars an scan and tag all enemies.
Stay by the gate and snipe them all.
If you move forward too fast, there will be an enemy on a machine gun, and he has perfect aim.
The main problem is to keep Val back, because she's dumb as a brick
and will run into enemies hand.
When the way is cleared be sure (!) to ckeck the building to the left and kill all remaining enemies so they won't kill you by surprise.
That's it, Val jumps on the forklift, drives a few meters, level finished...