I get why you're trying to finish up the multiplayer achievements first but it is better to at least power up one of your characters in single player before doing so. IIRC, you can't even join the later chapter areas if you don't complete those in single player. Plus it's easier to get Poetic Justice the stronger you are.
If you still need people for Boned (requires 6 people), let me know. I haven't gotten that yet.