Originally Posted by Misfit X7
As I don't currently have a Gold Membership...I'll be severely happy if i can get this stupid thing in undead Nightmare.
Can confirm - I got this last night in El Presidio playing Undead Nightmare. Still takes a while, but the undead just keep coming. I used the canon to the right of the gattling gun (looking out of the fort) and could choose targets from the left and right. Easy achievement and thanks to the folks here who led me to it!
I think the counter resets, so any kills from the SP campaign and multiplayer don't count, but you still get up to 500 pretty quick.