Pilot a helicopter in single player
You'll get this on the first level.
To Hell and Back 10G
Complete Mission 9 on any Difficulty in single player
Just finish the game (since this is the last level) and it's yours.
Goody Two Shoes 15G
Bring 20 Prisoners to the Church in single player
This is the secondary objective for the first level and will either result in you being Johnny on the Spot to prevent failing the mission or fail it in order to get the achievement (which ever comes first). They are indicated by the black arrow and are all over the place, and since you are timed you really have to move your ass to avoid failing the level. You have to do this in one go also.
*Has to completed before destroying the mortars.
A Mess of Stiff Staff Sergeants 15G
Kill Inferno's Sergeants in single player
This is the secondary objective for the second level. All but I think 2 of these guys are on your way to destroying your primary targets. They are indicated by the black arrow and are in lightly armored vehicles, making them easy to kill.
*Has to completed before destroying the commander that tries to escape in the helo.
Jonesing for Relics 15G
Return all the Relics to the Museum in single player
This is the secondary objective to the third level. There are what seems like a ton of these things but all are on your way to primary objective(s). Just pay attention to the black arrow and you shouldn't miss any.
*Has to completed before destroying Inferno's Warbird the second time.
From Cold Dead Hands 15G
Steal all Inferno's Weapon Crates in single player
This is the secondary objective to the fourth level. Like on the first mission you'll be runninjg out to get these and bring them back to the village, but to different places each time. There aren't as many to get this time though. Several are located at different points and a few are grouped together, but they are all in close proximity to one another. Just follow the black arrow while being as quick as you can and you shouldn't have a troubles.
*Has to completed before destroying the APC’s that attack the village.
Supply Problems 15G
Destroy all Mining Carts in the Kaabe Mountains in single player
This is the secondary objective to the sixth level. All of these are on your way to the primary objective and you will actually have to do the primary to continue on to get these. Just be mindful of the black arrow and you shouldn't miss any.
*Has to completed before you destroy the wall to get to Natasha in the refinery.
Inferno's Briefs 15G
Steal all the Document Briefs in single player
This is the secondary objective to the eighth level. Like on the third level you just have to get these and there are bunch of them, but they are all on your way to the primary objective. Again, as long as you pay attentionh to the black arrow you won't miss any.
These are all easier to get on normal by the way. Here's a written guide to all these secondary achievements and a walkthrough by mwinter over on GameFront (all credit goes to him/her), check it!
The Daily Grind 20G
Get a character to max level in single player
In order to get this you'll have to get at least one of the four characters to level 45. It sounds like a lot but its quite easy to rank up and you should be around 38 to 40 by the time you finish the game. So, you shouldn't have to grind on it too much. You get experience from destroying enemies and doing primary and secondary objectives. Remember, you can only gain levels by playing on normal or hardcore.
Note that some people are having problems with this, as their characters are resetting themselves once they get around level 35 to 40. I've got it and know of other people getting it so it's not impossible but it is more than likely glitched. You may want to wait for a patch/update or can attempt it at your own risk.