It looks like all but 6 of the achievements are meant for single player, offline play. Here We Come, Return To Sender, Rocking The Boat, Hide and Seek, Late Night Pick-Up, and Getting Even all require you to complete certain co-op maps, so they can be achieved through online OR offline co-op play.
Well, I can't really say I'm surprised that Far Cry 3's online multiplayer isn't good. While the game is unique in a lot of aspects, it reminds me a lot more of an RPG than it does a shooter. It just seems like non-shooter games never have good online multiplayer.
If you play the co-op mode offline, I'm pretty sure you can get ALL of the achievements through offline play. Like someone pointed out above, there are only about 5 or 6 achievements for co-op play, and all the rest of them deal with single player mode. The game doesn't have any achievements specifically for online multiplayer mode.