When you boot up your game, at a random location a hidden door will spawn. when you find and go through the door you will be confronted with a safe. the code is as follows:

once you enter the code with will obtain two items. the "Meth Lab Key" and the "Office Key"

Once you have the required items you can move on to the second phase, locating the office. it spawns randomly as well. the office door can spawn anywhere in level zero. it appears as a brown wooden door with a gold knob. to unlock the office select the "Office Key" and use the key on the door. after you enter the office you will be presented with two paths, take the nearest door. this room is the lab entrance, from here use the "Meth Lab Key" on the door and enter the room. take the "Bag of Meth" item from the table and exit the room.

Now that you have the required item, you can roam as you wish or summon Walter to defeat him. simply use the item on him and he will thank you and teleport away. and that's it! you beat the Walter White mini-boss!