This is one of the shortest and easiest levels in the game. There are hardly any ghosts to fight, except for a fairly major battle against Chef Deforrest and his goons in the kitchen.
The level is pretty straightforward as you will need to turn on the power with a generator behind the kitchen. You will need to use your slime tether to remove the power line from the water. As you leave the kitchen area, you will be attacked by the Chef.
After the power has been turned on, don't forget to grab this achievement just off the main hallway.
Once you go up the elevator with Egon, it's pretty straightforward to get to the end boss. A few turns here and there will get you there. The only enemies are small spiders. Keep an eye out for cursed artifacts.
You get to room 1221 by revealing the door with your P.K.E. meter.
The boss is the Spiderwitch, whom you've seen through the level. She is not particularly difficult to defeat. Scan her with the P.K.E. and it will say the Proton Stream is her weakness, but I've found that the Shock Blast to be quite effective. When her energy gets low, she will disappear to feed and recharge her energy. Find her with the P.K.E. meter as fast as possible to keep her from recharging. I have found her most often in the middle of the lair. If she is glowing red, that means she has recharged some of her energy.
Defeat her and return to the normal world and the achievement will pop.