MAIN THREAT: HA HA HA! None :)
This level is just as easy as the first, but is slightly larger. After you have collected the Shield and Energy Boosts in the first room, you'll enter a large room with a pack of C1D's waiting for you. After removing them (by laser), have a look around. From the viewpoint when you came into this 2-layer room, there are doors on all sides of the room, all but 2 of which can be done in any order. The one below the door to the first room leads to the Blue Door, while the one below and to your right is the Red Door. I'm just going to pick an order in which to do the 3 remaining doors, for the sake of the walkthrough.
Going through either of the two doors directly ahead, you'll run into some C1D's and a few C2D's. Keep going straight ahead until you reach an intersection with 2 Energy Boosts. Head straight and around the corner, where you'll find several SMH's, C1D's, and C2D's guarding the BAK. Destroy them, and then head down the tunnel below the one you got there by. This passage leads a room with C1D's, C2D's, and the hostage room.
After collecting the hostages, head back to the intersection where the Energy Boosts were, and head up either passage, turning at the first intersection. You'll come to a dark room with several tunnels connected to it, along with a few Drones. After clearing the area, head down the right-hand passage of the forked tunnel (it's blue). The tunnel leads to a room with a couple of C1D's and two other doors, along with an obnoxious ceiling bulge. The tunnel with the yellow hazard stripes on it has a door to a room with some Proximity Bombs. There are also two secret doors here, clearly visible on the wall opposite the remaining door; it would be a good idea to grab the Laser Cannon that's in the left-hand one.
Head through the remaining door, and note the two tunnels that go off to either side of this next room just before the tunnel to the door. The left goes back to the multi-tunneled room, and the right appears to be a dead end, but has a hidden door in the left wall leading to another set of rooms. Watch out for the C2D's upon entering, and in the next room there are some more C2D's, two SMH's, and a Vulcan Cannon. There are also three hidden doors, one on the left side of the room (Shield Boosts), and two interconnected ones on the right side. One of the latter two has 2 C2D's hidden behind it. The other has another Laser Cannon.
Backtrack to the dead-end passage, and head through the door at the end of the tunnel below. You'll be back in the large two-layered room, and will want to head for the lower door on your right. Inside this door are two C1D's off to the sides; nab them and go through the Blue Door, where you'll be greeted by a C1D at the top of a long shaft containing 5 C2D's. At the bottom of the shaft is a small room with 2 tunnels off to the sides (wrapping around the bottom of the room), leading to an EC and Shield Boost, a wider passage straight ahead, and a pack of Drones. Pick them off, go through the EC if needed, and head for the wide passage. There are C2D's in both directions, and the room the tunnels meet at has 3 SMH's (one guarding the RAK) and another C2D. Wipe them out, grab the Key, and backtrack to the two-layered room (stopping at the EC if you'd like).
From the main room, head through the Red Door. In the first room, there is a C2D waiting, and the Exit is in the ceiling of this room. Note the two hidden doors off to either side--these will open when the Reactor blows. In the next room is another C2D; blast it and pick up the Laser Cannon (you should have 4 now). Through the next door is the Reactor room. In here are 6 SMH's, along with three hidden doors (two off to either side of the pit (Homing Missiles), and one in back with a Cloaking Device). Shouldn't be too hard to blast them to pieces (grab the Cloak if you want) and blow the Reactor. Once that's done, head back through the previous room to the room with the Exit. The two hidden doors have by now opened to reveal two SMH's and a Shield Boost. Nail 'em, and scram outta there.
- In a wall in the room with the ceiling bulge; contains a Laser Cannon.
- Two cubes over from the previous passage; has an Energy Boost.
- In the left wall of the dead-end passage; leads to a room with a Vulcan Cannon.
- In the left wall of the Vulcan Cannon room; has some Shield Boosts.
- The left-hand hidden door in the right wall of the same room; has two C2D's and some Concussions.
- The right-hand hidden door in the same wall; has a Laser Cannon.
- In the back wall of the left-hand passage; has two Vulcan Ammos and an Energy Boost.
- In the back wall of the right-hand passage; leads to same.
- In a wall on the far left end of the Reactor room; contains a Homing Missile Pack.
- In a wall on the far right side of the same room; has another Homing Missile Pack.
- In the back wall of the same room; has a Cloaking Device.
- In a wall on one side of the room below the Exit; contains a SMH and a Shield Boost.
- In a wall on the other side of the same room; has another SMH.
- EXIT: In the first room inside the Red Door.