Don't use the ladder. Instead, take your time to explore the surroundings. You'll probably notice a few soldiers on the lower level. I'd strongly recommend using a crossbow to get rid of them, especially since they won't expect your attack. You can also go back to the small room if you have lost too much health.
A second group will appear near your current position (screen), so you must be very careful while trying to kill the soldiers. Focus all of your efforts on neutralizing them. You should also be aiming at the gas canisters in order to get rid of them quickly. Once they're gone, head on to the room they where hiding in. You'll find an ammo crate there.
Make sure that you've been able to secure this entire area. Once this is done, go back to the ladder and use it to get down to the lower level. There are some new balconies here and you'll have to use one of them to reach your current destination. I would recommend going right. Use your crossbow, because there's a single enemy unit in front of you. Move closer to a large construction. Save your game.
You are going to be attacked in just a few seconds. I'd strongly recommend using a slow-motion feature here. This should allow to find a good cover. Make sure that you aren't exposed too much to enemy fire and start shooting at the soldiers. Use a crossbow or a Hell-Fire machine gun. Your main task here will be to get rid of all enemy units.
Now you should consider moving to your left (screen). You will probably notice two snipers and it's up to you to kill them. You must be extremely careful here, because they're well equipped and trained. Consider using one of your powers to your advantage. As for the weapon, you should choose a crossbow. Once they're gone, save your current progress.
Your next task here will be to find a large wheel. Look at the screen if you're having some difficulties figuring out which object it its. You will have to solve a very simple puzzle here. Press and hold the X key in order to turn the wheel. Wait for an elevator to show up. Once this is done, activate a time stop function and start running towards the elevator. Disable this mode once you're standing on it. Wait for the elevator to get you to a new floor. You will also be granted with a new challenge.
New mission objective: Locate the vent shaft that leads to the plant's central core
Once you've reached your new destination, kill one of the engineers. He will be standing to your right, but a new group of enemy soldiers will approach your position from the left. Take them out as well. Remember that you won't have to leave this are right away. It's best to wait for them to reveal their positions. There are also gas canisters in this area. It would be wise to use them to your advantage. Save your game here.
Now you must find a long platform which leads to a new section of the factory. Look at the screen in order to find out which one it's going to be. Try moving forward, however once you've spotted enemy troops, move back to your original location. Take out all enemy soldiers from here. Obviously you will have to use a crossbow or a sniper rifle, because there's a large distance separating you from each other. Make sure the area is clear and start moving forward.