For the overkill achievement, you need to be in Lone Wolves. Anyways, my own little story. I was in Lone Wolves and it was FFA Swords on Guardian. Two people had 24 and I have 18, so I was third. Somehow I managed to pull a kill, a double, and then a triple, by making my way from gold lift to sniper spawn, and then I dropped down and assassinated someone for my triple kill, overkill, stepping razor, and that beat down achievement, all before the top two people got a kill. The game finished with me a close contender for the lead. xD Anyways, I still need Cortana metagame (I can't do it with my second controller turned on and afk - the Flood that always chase me end up mauling the arbiter instead when he teleports.) I also need the killing frenzy, Two for One, Mongoose mowdown, and some other BS ones.
My best achevment ive gotten so far was steppen razor but i just dont want to do meta game and beat it on legendary im so lazy