Thanks to this months pauper game of the month sessions, there are a fair few playing FC2. There are infinite numbers of guides on how to do all the achievements, however this brief guide should hopefully save you from having to play through the game multiple times.

The game uses a primary, secondary and special weapon system where you can only have one weapon of each type at any given point. This means that if you are daft enough you can go out on a mission carrying odd and unfunctional combinations. My recommendation would be a sniper rifle for your primary, and the pkm for your special weapon. Secondary weapon doesn't really make a difference. There are also silenced weapons, but from my experience apart from the fact they don't make a sound, there's no difference - enemies still spot you really quickly.

Use the boat. There aren't many checkpoints on the river, so it really is quicker to get to the rough area you want by boat and then use a car or go on foot to finish off. If you prefer walking then the camo suit upgrade is worth the money. If you are having trouble navigating past guard posts, it is much easier at night.

Order of play:
It is possible to miss collectables in this game, so my suggested order of play is
  1. Collectables - buddies, diamonds, tapes and safehouses.
  2. Arms dealer missions (you might want to do one or two early on to unlock some better weapons
  3. Communication towers
  4. Buddy missions
  5. Underground missions
  6. Main APR/UFLL missions (making sure to subvert them for your buddy)

This link has an interactive map that you can mark off the safehouses, diamonds and tapes from. There is no need to mark off the communication intercept missions as they can be picked up from any tower.

If you follow this order then you should have all the diamonds and have handed in all the tapes to the reporter (who is in Mikes Bar if you forgot) long before you travel to the south section of the country (which is when some of the collectables become unavailable).

Remember to talk to your buddies in the safehouses so that they will back you up if you get killed. And when collecting save at every available point you come across - especially before collecting any of the diamonds using the hang glider.