This demo shows complete handling of the red area of level 11, including freeing yourself from the "appearing wall" trap and exposing the reactor.
It begins outside the red door. The first order of business is to dispatch the welcoming committee that waits behind. Once inside the door, I try to draw the Smelter, waiting beyond the lavafall, out of his hiding spot by making a commotion. Sometimes he comes out, sometimes he doesn't. This particular time, he didn't, and I had to go in after him.
Before descending into the next room, I drop off some gifts for the surprise party that awaits me on my way out.
Also, I clear said room in a bit of a wasteful fashion. Two Smart Missiles, and the Seeker that lurks below is history. I do this so as not to waste time during the demo. I would prefer to save at least one of those Smarties for later, but it would require a lot of patience and boring tactics.
Now, it's time to enter this room, where the first thing I do is remove the fence covering the lava pit, so I have a clear shot at the resistance found underneath. Then I clear the far room. Turns out this is the reactor room.
When you enter this far room, you become trapped by a wall that appears behind you. [Aside: This is a bit weird, because it's not the way I remember it. I recall being able to slide into the reactor room along either wall without becoming trapped. Perhaps this was changed in one of the patches.] I free myself by hugging the wall and entering the robot generator on the right. This does not trip the generator. When I exit the generator the same way I came in, the entrapping wall disappears.
But where is the reactor? It is hidden behind the wall above the entrance to the room. I remove it by tripping the switch in the far corner. It is very hard to see, and I will trip the generators if I move in to get a close look, so I do it from a distance. (OK, so this isn't the first time I've been in this room. :-)
Once the reactor is exposed, it's just a matter of destroying it while hugging the wall so as not to trip the generators, then exiting the mine. My gifts are appreciated by the hosts of the surprise party, so the path to the exit is clear.