The Observer is a bossfight found and set in the Legacy map. By beating it, you get the Mighty weapon skin and the Dimensional Conqueror achievement.

To start the event, you must swim under the walkway by the water, around where it protrudes forward.


Then, you must hug the wall until you find the part of it that is interactable. Keep in mind that this area has no pipes.


In order to beat the bossfight, you must find 10 papers (like slender), bring them to the top of the central tower, craft the RCV2 using the desk (where the bat usually spawns), and then delete the Observer. There are also Mighty M1911s that spawn across the map that can stun the Observer momentarily, though it takes a few shots.

Spawn a treadmill from Static Props > Interior, as well as a crate or something else to block the back end of it.
Then, just stand on the treadmill and leave it as the speed slowly increases. You're going to want a speed of at least 2.00, as this is more than enough to outrun the Observer just by running alone.

Additionally, you may also want to train the endurance stat by holding a dumbbell. I got mine up to 20, and when I tested it, I was basically immune to the Observer. I don't know if you can do this at the same time as the treadmill, but it is something to keep in mind.

You may also want to arrange the benches that spawn naturally like this to save time.
This should save some time getting the first paper, though make sure you walk up the benches because of the slip function.

Finally, you'll also want to delete all the doors in GDev's hotel, just to make it easier to traverse. I would recommend opening the garage door too, because the speed at which you will be running makes it difficult to get through the main doors.
Remember to open the basement door via the button in this room.

Here are where the papers and pistols are:
The red marks are where the papers are, and the green marks are where the M1911s are.

Literally just run around and get these all as fast as you can. You'll be running so much faster than the Observer and barely take any damage from its hits (if you train endurance) that you should get through the fight pretty easily.
You'll want to get the paper on the main tower last, as this is closest to the crafting bench you need to make the RCV2.

still waiting on observer spawnable nextbot