-Estimated achievement difficulty: 3/10
-Offline: 35 
-Online: 15 
-Approximate amount of time to 1000: 15-20 Hours SP, 10 Hours Multiplayer [25-30 Hours Total]
-Minimum number of playthroughs needed: 1 [Can keep playing game after to finish]
-Number of missable achievements: 1, ["Chatterbox", but others can be much harder to gain after the game is complete]
-Do cheat codes disable achievements? No cheats.
-Does difficulty affect achievements? No [Can play on easy]
-Glitchy achievements: None. Though some normal glitches are present. You may want a back up save
-Unobtainable achievements? None
-Extra equipment needed? No
Online Pass Required?: No
Important Note: This game has a habit of crashing for no known reason. If the game crashes during an autosave, which happens frequently, your save file will become corrupted and unusable, meaning you'll need to start the entire game over again from the beginning. There is no known way to avoid the game crashes (despite the developers having put a patch in place to improve crashing and how the game handles it), nor is there a way to recover your game from a corrupted file. There is, however, one important thing you can do to ensure you don't have to start over if your game crashes and corrupts your save file: each time you save at a base camp, after saving, go back to the base camp and switch save slots to a second slot, and let the game save there too. To be safe, you can repeat for the third slot as well. This ensures you always have a backup save or two in case one gets corrupted. It may seem tedious to do it at each base camp, and you certainly don't need to save three times every time, but the risk of having to start an entirely new game is always a possibility if your game crashes. Better safe than sorry.
Though this is an action/platforming game involving a lot of jumping/climbing mixed with a few puzzles, you will have no difficult time completing the game. It is easier compared to other games that are similar. You can play on Easy combat starting out, or change it to that later. There are very few puzzles which are "optional tombs", but even those can take as little as 5 minutes to complete. The big bulk of the single player is keeping track of how you are getting kills, looting everything you can, and later going back to collect anything you missed. Yes, there is the ability in this game to go back after completing via fast travel locations. Be warned, that when you reload a checkpoint or die, none of the stats save for your kill types. If you grab a collectible, however, you will not have to get it again. There is one major missable achievement; "Chatterbox". Check the guide, or just make sure after every scene with a Survivor, you talk to everyone around. If you fail to do so, you will have to play the entire game again and it's only worth 5 points. As far as collectibles, you just have to remember to always press to see things highlighted in gold. You will gain skills to help for these as well, and they will also show up on your map eventually. Therefore, there is really no guided need (thankfully).
Step 1: Story
Play the game on any difficult you want. If you want to enjoy the game, just focus on getting the kill related and misc achievements. As far as collectibles and challenges go, grab what you want and move on. You will not be able to always grab everything in the area. This is because you get different tools/weapons as you progress that help you get the collectibles. After you have done your playthrough, you will be able to continue and finish getting everything. There are even some enemies that respawn.
Step 2: Finish What you Started
Now that you're fully upgraded and have all fast travels unlocked, go back and clean up all the stuff you left behind. Finish your weapon modifications and get the rest of your skills.
Step 3: Multiplayer
Most all of the multiplayer achievements can be boosted in private matches. Even with ranked you can sort of boost because you can be in parties. You will need a ton of experience to be able to buy all character models and upgrades. This will require salvage. As you make your way to Level 60, keep in mind that you must break the boxes apart to gain extra salvage. There is even a perk that allows you more. Make sure you have it on. You can boost your level/salvage in private matches by yourself playing Rescue (it's like capture the flag). Make sure you are the Survivors so that you can go first, then quit.
Use the Achievement Trading Thread to find people.
This game is extremely easy and has breathtaking visuals and superior gameplay. The mechanics work extremely well too. There is a bit of grinding for multiplayer achievements, but in the end you will have a completion without much effort.
WARNING MULTIPLAYER UPDATE COMING
Good news, the General will soon be free and not require you to prestige 3 times and get to level 58. Do not prestige, stay at your level.
Visit the thread below to learn how to buy the General early before the update.
Check this helpful thread
[XBA would like to thank Tyger7 for this roadmap]