Call me crazy but I think I've finally managed to get that FFA grenade-launcher Co-Op method to work FFA, 20 minute round and 100 kills on Underground. Literally followed the video in the OP exactly and my friend and I were getting 70 kills each for anywhere between 98-105k every game!
I just don't understand how they're doing it. Every time I try that method I'm only getting 70k tops. What gives But I'm gonna continue to try it out tonight, if I manage to get the 100k+ XP, I'll let ya know.