Well, I've come to the conclusion that I can't get all the achievements I need by myself. I'm trying to beat the game on legendary and get the meta-game achievements as well (not at the same time, mind you). In the meta-game, the Arbiter and marines take too many of my kills. And I can't get any points unless I am the gunner in the vehicle and my marines drive like they just got their license. And legendary solo? Forget about it. The Flood make me look like a chump.
So if anyone is interested in going for these achievements in co-op, let me know. I'm trying to get the security armor. Also, I have a weird compunction where I have to have everything a game offers or I feel like I've been cheated...
My gamertag is: three50 TheSTiG
Add me and send me a message if you want to get the achievements or if you just want a friend for Live play.