In the Guardhouse, when you walk past the Guardhouse Save Room, you will notice a nearby area with holes in the floor! Do not walk over them or one of Plant 42's vines will attack you! Push the lone box which is near the Double-Doors at the end of the hall over the hole in the floor which is in front of the boxes in that general area. From then until you defeat Plant 42, use the box staircase to get to the door beyond the holes in the floor!