For "Real Nasty Habit", there used to be a proxy exploit that glitched the game into giving you XP for kills you got in the Single Player game. Unforunately, they finally patched it out with the last PSN 4.40 update
However, you can still get it by playing Survivor, so that's something at least.
Six Pack is a pain in the ass to get done, simply because the matchmaking is fundamentally broken as well. I spent, I kid you not, about a month straight trying to get a group together for it. Finally did it, and there was QUITE a bit of cheering done over the mics.
Feel free to add me on PSN if you need the headcount.
id: Nuclearporkchop