Sorry if this has been covered, but I just heard about them. However I'm having trouble with Youtube over here, so I can't see any vid explaining how to get them. Can you guys help ? So far from what i know is. Complete all steps of the egg up unto you have the Elemental fist. Then go inside a robot, down your self with a frag and then what ? Also, can this be done in solo ? Any help on this would be great. I'm not working this weekend and this seems like it could be some fun to do.
There are videos on youtube if you search 'elemental knife origins'. The first two videos are from glitching channels and advertise it as a glitch. It can only be done co-op, not solo, but I'm pretty sure it's not a genuine upgrade and is not meant to be in the game.
It can't be done in solo, as you have to bleed out to get it. After you bleed out in robot's head and respawn next round you will have elemental knife. If you bleed out with elemental knife you will lose it. To get it back you have to get fists from chest and bleed out again in robot's head. Personally never used it (I consider it a glitch, also I don't like to die) but it's getting pretty common in public games.