In giantess mode, you need to get the card from the mess hall then go back to the first room with the guards and go in the door around the corner and turn off the power then the door with the red light won't alert the guards. Make sure you shut the door behind you each time you go through the main door. There's a camera screen on the inside of the main door so you can check where the guards are when you exit. Hope this helps. =)