***UPDATE: Patch 3.0 from 26/09, grenade launcher ammo now can be refilled only with the Ammo Boxes and can not be refilled from the infinite Ammo Cache.
Making this solution obsolete. Maybe sitting on the ammo crate is still a good strat, but obviously slower, to get kills with normal weapons. I will leave the solution for now since there is no other guide***I found a very good spot on Sky Fire campaign mission without reloading checkpoint, sitting on an ammo box in a mini glitch, almost safe from all xenos attacks and spamming grenade launcher on infinite waves. (around 200-250 kills / minute)
First you need to pick up the bolter grenade launcher behind the tree in the very beginning of the jungle section.
Once you arrives at the door defense, make sure to focus on the bigger xenos and the ranged weapons ones. They wont spawn anymore afterward.
When the door open you can position yourself on the ammo box like i show in my video.
Make sure to focus on the wave coming in the pit before shooting the ones around you to make the next wave spawn faster.
The rampage can start, have fun
Here my video showing where to find the bolter grenade launcher and how to get on the ammo box