I found this a while ago. It can be done with the bushwacka, , rtd(Rolls like Low Gravity, Lucky Sandvich, Godmode and Infinite Double Jumps), stickies or sentries.
1. When you spawn, go to any of the pipes
2. See that wall?, It looks transparent, but you can climb it using the bushwacka or you can use your stickies(You will need a medic, lucky sandvich or godmode). I'm going to use the stickies
3. When using the bushwacka, hold W so you can get to the top of the wall. With the stickies, you will notice when you're on top.
4. If you move towards the gap, you will fall under the map.
5. Once you're there, you can shoot or set up a nest. Only teleporting bosses or bosses with huge superjump will be able to get you.
A video using the stickies and lucky sandvich. https://www.youtube.com/watch?v=F7RAJM0ZhpE