-Estimated achievement difficulty: 6/10
-Offline: 66 (1475)
-Online: 2 (75) (Defender, Master and Commander)
-Approximate amount of time to 1000: 68+ Hours
-Minimum number of playthroughs needed: 1 (2 if not importing save)
-Number of missable achievements: There is no replay of chapters, so you want to get all the side-quests done in one playthrough. The major ones are Paramour, Saboteur and Liberator. Also watch for missable achievements in the single-player DLC; Freedom Fighter in the From Ashes DLC, Conspiracy Theorist from the Leviathon DLC and Meticulous from the Omega DLC.
-Do cheat codes disable achievements? No cheats
-Does difficulty affect achievements? Yes (Insanity Difficulty)
-Glitchy achievements: None
-Unobtainable achievements? None
-Extra equipment needed? Kinect can be used for voice control (optional). What is required to do multiplayer is the Online Pass. So if this is a rental, you will be alone.
-Is there DLC? Yes
You can play Mass Effect 3 from scratch if you have never played the previous games. If you have a save file from a previous Mass Effect, you will be able to import your character and save yourself an entire playthrough. This game has a lot of story-related and kill-based achievements with a few other random ones. There is only one basic difficulty achievement, which is for Insanity. There are no collectible based achievements in this game, so you can cross that off your stress list. Also, in the main menu, you are able to look up your achievements and see a tracker that shows how many you have towards that achievement. It will also pop up your status as you play, so you never have to look at it.
Step 1a (New Game): New Game on Easy/Narrative:
If you're new to Mass Effect and are starting from scratch, that's perfectly fine! The game does not require you to have played before. You can play on easy to get the hang of things and build up your level before moving on to a 2nd game. This playthrough is up to you. You can even put the difficulty on "narrative" which is easy mode to the extreme.
Step 1b (Imported Character): Insanity Playthrough:
If you have imported a save from a previous Mass Effect, you only have to beat the game once. You should be gaining all of the campaign based achievements.
Step 2 (New Game): Insanity Playthrough:
After playing the game for the first time, you are ready for Insanity. You'll have to be more patient while fighting in this playthrough. Make use of your powers and squad as much as you can.
Step 3: Multiplayer/N7 Missions
There are a few achievements that can be done easier online, such as using some of the powers that your single player character may not have (especially if you are a Soldier). It is also much easier to max your level online and be promoted than to get max level in the campaign. However, doing the N7 Missions on Insanity is probably easier than finishing all the MP maps on Gold unless you have a few high-levels friends to help.
Step 4: Mop Up
At this point if you only did one playthrough and did not get a level 10 weapon online, you'll want to do a NG+ and finish Gunsmith, as well as any other power or kill-based achievements you may have missed.
Step 5: From Ashes, Leviathan and Omega DLC
These story missions are easy as far as achievements are concerned. Other than missable achievements such as Freedom Fighter (From Ashes DLC), Conspiracy Theorist (Leviathon DLC) and Meticulous (Omega DLC), you can focus on simply enjoying the story and gameplay.
Step 6: Citadel DLC
First you should proceed to follow the main story of the DLC. You will earn achievements along the way, for completing certain missions, which of course makes these unmissable. Completing the story will earn you 4 achievements, worth a total of 125. After you have completed the story, you can then proceed to complete the miscellaneous achievements, that you can earn at your leisure. There are 5 remaining achievements that you can earn and they are all easy and straight forward. The first 2 are for throwing a party for your friends and to win a prize in the arcade, both of which are straight forward and should pose you no problem.
The remaining 3 are all earned in the combat simulator. 1 of these is for fulfilling a request within the simulator, a sort of mini-mission. The other 2 are for reaching certain goals within the simulator, but you can choose whether to do them on normal, hardcore or insanity difficulty, so you shouldn't struggle with these as you can do them on normal and breeze right through.
Aside from the difficulty of Insane and Gold online, the achievements are all fairly simple here. Be wary of the timed side-quests (see the link in "Unwavering") and you'll be set for your 1550! Also watch for the missable achievements in Leviathan and Omega.
[x360a would like to thank Tyger7 for this road map]
[x360a would like to thank Asturgis and Scotty for the DLC additions]