If you want to get level 150 as fast as possible i have figured out the best way. Which is have 1 friend as your sidekick and one as the afk monster.

You should play residential for stage 1 and cryonics for stage 2. Because from spawn there are 2 easy switches. For the first switch look for this pool table in the downstairs area.

It should be close to your spawn depending on the rng of your spawn location. You just need a welder for the pool table switch just pull the forklift jack backwards and use the welder tool on that box that will now appear in the ground.
For the second switch go to this room then once you have made your way inside that room in the picture below.
Look for a button that looks like the one in the picture below. After pressing that button a white screen should come down from the ceiling.
Take that code its random every time btw and put it into the keypad that is lit up in blue pretty close after the right code has been entered. The door will open which will reveal another switch.
Just have your friend do some other switch or switches it doesn't really matter which one/ones. I was able to finish 1-2 before my friend managed to finish 1 switch.

When you have completed another switch make your way to the cryonics. When you have gotten to the next stage look for a welder symbol if you cannot find one.

I recommend the room with forklift jacks you have to look either under or behind the jacks and you might see the code.
I recommend doing the welder always mainly because it will give 450xp per welded pipe but it does not always spawn.

When you have finished whatever cryonic quest you chose go to the submarine and follow that objective. If you're lucky the submarine also has pipes you gotta weld.

I will drop a youtube video below to make my method abit easier to follow its not my fastest due to multiple mistakes.

If you manage to get the perfect rng you can easily get sub 3 minutes with 2 people. Shave off a minute then you got your duo time.

Just make sure you do not I REPEAT DO NOT TAKE DAMAGE AT ALL if you do you will not get flawless bonus which is 6k xp every game.