Quote: "Well just make her appear for like 2 secs with flickering lights, fire and explosions with some crazy sound... THEY WILL FREAK!"
No they won't, cheap tricks like that are ten-a-penny and don't work...
You don't want a huge BWAH! style scare, you want a long, drawn-out, suspense-building sequence. A sequence that makes the player not want to go through to the next room for fear that something will attack them...
You build and build and build it, then cease it to create a false sense of security, then you BWAH.