Alright, here it is!
http://www.youtube.com/watch?v=GCsZv...ature=youtu.be
Go to big stone reach and find the mission "Saving Echelon". Save the four mercs, then go to the hole in the northern wall, wait there for two and a half minutes, and then EGO will say "Careful! We've got a lot more hellbugs on their way!"
At this point, a wave of 10 skitterlings will spawn, kill them, and they respawn a few seconds later.
I will note, that weapon experience in based on damage done, and that overkill damage counts to weapon experience. Dealing 2,000 damage to 10 skitterlings will give 20,000 weapon exp. As a result, Stack most of the +ego and +explosive perks at once, respec if necessary. You should be able to keep overcharge constantly going as well as dealing 2,000 damage or more per shot.
This works best with explosive weapons (some explosive weapons don't benefit from explosive perks like the clustershot or courier, even though they are still the best ways to level those weapon types)
The following weapons work best for each skill type. You mainly want high damage with a large explosive radius.
Launchers - Rocket pod, Longshot Cannon, Armada
Detonators - Big boomer, Grenade launcher
Combat Shotguns - Clustershot
Sawed-off Shotguns - Courier