This level is very easy until the end. Just keep the cattle prod equipped, watch for raptors as you progress, and let them have it. Be careful with raptors on an upward slope. The rule for navigating this level is "up and left". That means always advance upward when you can, and always advance leftward when you have a choice between going left or right. When you have a choice between left and up, go up. Assuming you don't deviate from this advice, you will always find the exit, although you will skip a few ammo and health pickups (you shouldn't need them with a little practice). When you come across a couple of medkits, you're probably near the end. You should then see a big control panel with a sign that says "DANGER". When you walk past it, the gas will activate. Equip the shotgun and keep moving to the right! Your health will drain as the poison gas takes its effect on you, and there's a countdown to boot. You will come to a drop with a ladder. Fall down and grab the ladder while you're falling to save time. (It goes down a little ways, but better to grab the ladder too soon than too late.)
When you reach the bottom, you'll come across a raptor. Quickly fire three shots of the shotgun to dispatch it. You can stop for a second, but not too long. Repeat for two more raptors. Climb the ladder, kill the green raptor with one shot from the shotgun, keep going right, then go down the ladder and jump over the compy below. Keep moving! The next ladder is the last one. Jump on it, move into the tunnel, and advance all the way to the right. If you're good, you should still have a little health and about 10 seconds on the clock.