Nothing in life is easy; but when you work hard for that which you enjoy, it seems easy.
I'm busy with something else but this is how I did it. On Snowbound Crazy King; I stood on top of the Guardian Gun above the Ghost / Mongoose tunnel in the cliff facing out towards the open area and kept my eye out for an opportunity while charging my laser constantly when I thought one was coming up. There actually was someone constantly intentionally stopping me from getting it throughout the game, but I still managed. I believe I have a short film of it on my file share titled "BOUT TIME!!!!!", if not I'll try putting it there.
Overkill I did on The Pit Crazy King. I got on the turrent and gun down everyone else trying to go for the hill. It is actually quite easy if done right and one could possibly get their Killing Frenzy at in the same session almost as easy also. I hope these suggestions help. Good luck.
[Edited on 09.18.2010 7:07 PM PDT]