I got mine orriginally on the Portuguese servers, because I just could't wait for my 1000/1000. However, in Lone Wolves, I've gotten three overkills since. All three were on Oddball on Snowbound. I grabbed the ball and ran underground, with everyone following me (make sure they follow you underground, it's important). I ran through a shield door into a base, and they all kept running. They didn't run through the shield door after me, I had to run out, kill one, and run back, but they were all lined up. Make sure all the players follow you downstairs, otherwise some people can sneak behind you. Watch your radar just in case.
I just got mine 2 days ago. I was so happy, even though it was just 5 gs. I got it on Epitaph after assasinating someone while invisible. After getting 4 eraneous kills, i finally picked up his shotty (only 4 shots left!) and got the last kill literally a milisecond before getting blown to hell by a grenade. The game didn't even show my medal onscreen, and it wasn't for 30 seconds afterwards that it even realized i had gotton the achievement.
i got mine on avalance wit an extermination lol...i no-scoped a dude bhind me took his shotty went thro portal n a noob followed me n i killed him went bak thro portal stuck a dude wit my sniper took it n sniped a dude off a hornet lol. then just made some sick kills getting Killpocalypse or watevr lol some of it was funny. the dude driving hornet got sniped then i sniped the other dude and got a tank n hog and all that. this acheievement is kinda hard i would do it the 1st time i have gold since u will play noobs :)