Flood's and Universe's Warhammer 40k fanatic and the one who knows much about it.
I also like House Stark and winter, hurr.
Camping. I got killing frenzy by camping in the corner of the pit with a shotgun and sniper rifle. As long as you will have power weapons along you and camp at some places for like 3 kills and then move on to other camping area, you should be able to get it. Cheap? Yep, but I think its the only realistic way to get it fast.
At overkill I cant help you, but with good playing and luck you'll achieve it.
Btw, I have once got untouchable and perfetion in Lone wolves. It aint that impossible, but requires bad opponents, power weapon whoring and cheap tactics to achieve. You will succeed if try hard enough :)
[Edited on 09.24.2010 10:09 AM PDT]