Most of it, really. There's around 50ish achievements (out of 600) that basically require boosting with people online, but there are many "multiplayer" achievements you can do yourself with another controller, and most of the achievements are campaign based.
75% probably
Cool! I'm going to start the entire Halo brother with my journey soon. I really really hope that I finish in time to get Halo 5 at launch, and I'm crossing my fingers for Halo Reach.