Stahl Arms Infiltration
You'll be back at the scene from the start of the game. Don't worry, you won't have to do the silly tutorial again. You will also hear instructions from Rico instead the silence of a fellow Helghast teammate. The first hurdle you'll come to is distracting the guards. Locate a valve on the left and use the Sixaxis to release the pressure. This will give you time to slip by and enter the next area.
You'll be armed with an experimental Helghast weapon--the Arc Cannon. It's very powerful and can kill any normal enemy in two shots. You'll still need to be a little careful as it can only fire about ten times before you'll need to find a dispenser. Enter the first room and take the far left hall. Rico and Narville may have trouble with the other forces, but don't risk your own neck to revive them. Often times you'll want to kill the large amount of forces before helping your allies. However, the Arc Cannon can take care of most of these guards.
Narville will separate to find other prisoners, so it will be back to just you and Rico. Conserve some Arc ammo by switching to a Helghast rifle. Clear out the enemies on the lower level of this floor and then take cover behind the terminals on the other end. More enemies will appear from the doors you entered from, but there is a dispenser nearby. Let loose with the Arc Cannon and restock at the dispenser when needed.
In the next room you'll be attacked by jetpack soldiers. If you reloaded the Arc Cannon feel free to go nuts and kill each enemy in two shots. Ascend up the stairs on the right and continue to abuse the Arc Cannon's fire power. An elevator to the far right will soon arrive with more soldiers, however once again blast with the Cannon.
There will be another long corridor, so take cover and pop out to use the Arc Cannon's charge to safely take out the Helghast herds. You'll reunite with Narville and the freed prisoners. Restock on ammo and take out the last of the guards before making your escape.
Once outside you won't be able to abuse the Arc Cannon due to a lack of dispensers. Use whatever shots you can and then jack a Helghast jetpack. You'll need to leap to avoid enemy fire and can use the building in the center to take cover. The final enemy is a flying craft that is loaded with rockets. Take cover and pop out only to fire your own jetpack missiles. After dozens of shots the craft will finally fall.
For the last stretch of the chapter you'll be in the ice saw. It automatically moves forward, but you can slightly steer its direct and aim. It's armed with both a machinegun and rockets, both with infinite ammo. Quickly blast the Helghan ice crafts and avoid bombs in the ground. As long as you are able to destroy the enemy vehicles you'll gain enough time to heal between encounters. Killing the enemy is more about self-survival, as the chapter will automatically end when you reach the finish line.