1. take a high recoil gun (sawed off)
2. Point it down
3. Jump and fire
4.Now flank on them poor people
5.(Optional) With the heavy barrel mod you can increase recoil causing a high jump
Ive been able to rocket jump but I have so far been unable to get forward momentum and I just end up going straight up then straight down
The desert eagle when duel wielded with heavy barrel works for this as well ( https://youtu.be/5m-IBG-runQ )
You can use the megagun( a very rare minigun varient) for this however its technique is just using it as a jetpack instead of as a second jump (video for reference https://drive.google.com/file/d/1An7PU2QUsesq4ej7bixFCmb5qpSzWsQ4/view?usp=sharing)