-Estimated achievement difficulty: 8/10
-Offline: 37 (1005)
-Online: 10 (245)
-Approximate amount of time to 1250g: 40+ hours
-Minimum number of playthroughs needed: 2
-Number of missable achievements: None!
-Glitched achievements: None!
-Do cheat codes disable achievements?: No cheats
-Does difficulty affect achievements?: Yes
This game is fairly easy to get the majority of the points in but for the full 1250 you will need to download two DLC packs (one of which is free) and also invest a significant amount of time online to get Retirement Savings Plan. The single player is pretty straightforward though and playing in co-op with a friend makes your life a lot easier.
Be aware that one of the achievements (Starting a Riot) does require you to play in human co-op, as the A.I. will never pick up the riot shield for you.
Army of Two has 1000 points available with the retail game, another 100 through the Challenger pack and a further 150 through the free sponsored DLC. You will have to play the game at least twice, as the Elite difficulty will not unlock until you complete the game for the first time. You can bypass this by playing online co-op with a friend who has accessed Elite difficulty already. Also, it is impossible to get all of the weapons kills in just one play-through as there simply are not enough enemies. You can just select individual levels in order to rack up the kills, overkills etc achievements though. Online is a different story as you will have to rack up $1 billion which is rather time consuming.
Step #1: Playthrough on Normal
Play through the game and focus on using each weapon until you have the required number of kills with it, then switch to another. Save all of your money to buy the achievement specific guns and use a guide to locate all of the briefcases on each level. It would be wise to do this difficulty with the A.I. as your partner for the achievement as they are less helpful on the highest difficulty. If you have the DLC then you can do the extra two missions as well, use the correct handgun to kill the last boss for an achievement here.
Step #2: Play-through on Elite
Complete the game again on Elite, again working on any kill achievements you are missing. Do this in co-op with a friend so you can get the Riot Shield achievement too, plus they will be much better help than the A.I.
Step #3: Clean-up
After finishing the game twice go back and mop up anything you missed, which will probably be the kills achievements and the DLC ones for destroying objects (only applies to the last two levels) and killing the last boss in under 40 seconds. You can just level select for any achievements you are missing here.
Step #4: Online
For online I would recommend finding three boosting partners as otherwise you are looking at hundreds of hours of legit play. Assuming you have three people then go and do the SSC Challenge matches first and take it in turns to get all the specific achievements and blow up the required number of barrels. Do not kill each other or buy any other weapons except the pack needed for an achievement and you will pick up pretty much everything. Once these are cleared up you will need to start playing twenty minute Bounties games in Afghanistan, taking it in turns for each team to do major and minor objectives. This will net both sides a large amount of money. Keep this up all the way to $1 billion.
NOTE: The online leaderboards for money reset every now and then but do not panic, your money earned up to that point will still be counted. The achievement willl still pop when your cumulative total is correct rather than it having to say $1 billion on the boards.
The single player is pretty much a walk in the park, though some of the DLC achievements can be a pain without a human partner. The online mode is a lot more troublesome, especially without boosting, and will require a lot of time and effort. I would suggest getting a dedicated team of people before even attempting the full 1250.