I've finally managed to earn the Nuclear title a couple of weeks ago and only need the challenges Chain killer and the one below it.
These were the ways I was thinking of trying for this challenge:
Ground War domination on a map like overflow or something where I can run circles around thier spawn area killing them.
Nuketown...rapid skill spawn camping from the side of the house?
I can't think of another way that would encounter this many threats needed other then the ways listed above. I suppose if spawns were wacky enough you could kill people spawning around you, but this is wishful thinking at best. And then you need to line up a game where your connection is not hindered and people won't quit because of it and the lopsidedness of lag compensation in general.
If anyone has gotten what in my eyes is the hardest challenge; how did you get it? xD