Walkthrough....
After the first dialogue, go forward and enter room 322. Open the upper drawer and you obtained the cafeteria food pass. Exit the room and keep going until you reach the cafeteria. Use the pass to receive food and give it to the man sitting at the table. He'll give you the key to the meeting room. Before you go, get a coin nearby. You need all three coins to get a soda from a vending machine. Then, proceed to the meeting room door and use the key from your inventory to unlock the door. Inside, you'll find a hammer (to get the shotgun) and a coin in the potted plants. Go to the door and use the hammer to obtain the shotgun. Get the third coin inside the toilet. Open the toilet bowl seat and then get the coin. Exit the toilet and shoot the padlock.
Proceed through the door and you'll face Ozmoze, the first boss and Alucard's cousin. In order to beat him, keep shooting him, but he'll attack you by telekinetically throw plates at you. Shoot them and you'll get the opportunity to shoot him.When you proceed towards the next scene,press the right button. After that, move on and proceed foward and you'll see Dick. Skip through the dialogue and press up. Grab the machine gun and the torch light. Go back to the Management room. Use the torchlight and click on the lower drawer. You'll find a paper for the code. (The code is 1.3.0.9). Enter the code and it would lead you to Hanzou. Killing him is easy if you have the machine gun.
After killing Hanzou, move on to the Janitor closet and you'll see Alien Hominid. Kill him to get the lazer gun and the key. Exit the room and move left. You'll see in the vent that there is something shiny. Shoot it and there is some shrooms. Eat it to heal yourself. Use the key to unlock the door to the rooftop and here will be a big battle.
To kill Damien, keep shooting his breasts. Shoot his antennas to disable the bolts. If he jumps, he'll create a force that'll blow you down, so press the up arrow key to avoid. After defeating Damien, a UFO appear after the short dialogue. It's easy to destroy and don't worry, it won't attack you. Then, after the last dialogue, credits roll in.
During the post-credits, you can watch the alternate ending and read to know why the game is delayed.