First of all, get on Aperture in a HoE long game with at least one teammate (make sure you BOTH have your level 6 Support Specialst equipped).
Now, you want to get to the end of the first wave. What you want to do is kill every zed and let a single clot live. (If you fail do to so on the first wave, no biggie! You can try again in the next one, it will just be a little less efficient)
Now comes the tricky part. You have to make the clot get bugged in the central room with the welded door, on the edge of one of the platforms (it doesn't matter which one, as long as you are in the central room). This is not as easy as it may seem and it's quite random, but you can cause this bug by making the clot follow you on the platform and then jumping down: he will try to replicate and sometimes he'll get stuck, as you can see in the picture.
At this point the clot will never move again but also he won't despawn since you'll always stay in the same room where he is.
Now you just pull out your trusted welder and you're ready to go!

Now you're getting closer to the objective.
The next step is learning how to perform the welder bug to hit your teammates with friendly fire.
This is surpisingly easy:

1)Start welding the big welded door with your teammate(s) (make sure you are close enough to each other to perform the bug)


2)Quickly look at your victim friend of choice while you are still welding


3)As you weld their body, you will hear their sweet screams of pain and there you go! Organized, super efficient friendly damage!


Note: While perhaps counterintuitive, unwelding your teammate won't heal them nor help them defending against further welder damage

At this point, all you have left to do is healing your friend:


Pull out your medical syringe, aim towards your teammate and...


Profit!



You'll be level 6 in no time!