As soon as you enter the level, switch to electrical view and select your Pistol. Walk over to the door in the wall (over to the left) and pull the lever next to it to open it. Walk forwards onto the lift and ride it down. When the door opens, fall down the gap into the area below Follow the corridor round until you come to a room containing some cylinders. Here, you want to go right. Go down the corridor and follow it round to the left, into another room full of cylinders. Worm your way through to the other side, where there's a lift. Pull the lever next to it to operate it. When you reach the top, head over to another lift, this time in the corner of the room. Walk along the ledge, then go right up the stairs and through the gap at the top. Go up the next flight of steps and then go round the corner. From here, head straight on to the next room. The door ahead is locked, but a lift will rise up to your right. Ride it down, then fall to the floor as far as you can go. Now switch to thermal view, cloak and sect your Speargun. Walk slowly towards the door, and when it opens shoot the Smartgun marine behind it. Follow the corridor round to the left, disposing of any marines you meet. Now you want to cloak and select your Speargun again. Open the door, and then stay still. There's a guy, usually on top of the APC, armed with a Sadar, and it will kill you in one shot. Find him and kill him, then empty the area of marines and enter the APC. Kill the marine in there to end the mission