When first encountering the Thing Boss, hold UP/LEFT to run diagonally and you should run under it before it can hit you. This will also aim you directly at the junction box. Immediately start fixing it. Once it is almost fixed, hold Right to run through the door. Since he will still keep fixing it as long as you do not release the button, with luck it will be fixed before the Boss attacks for the third time. At the intermission sequence you tend to take off in a random direction, so this may take a few attempts -- save immediately before trying this. If successfully completed, you will lose only about half a bar of health and no ammo, making the next area much easier.
Hang back in the back left corner of the room where you are out of reach from the Boss. Wait there patiently, and after awhile the Boss will knock out the lights. A bit later, he will lean over to the right of the room, as though inspecting something there. Choose this moment when he is leaning to the right to make a run for the switch you need to fix to get through the door. You can get by without too much loss of health (less than half).