- Estimated achievement difficulty: 5/10 [Achievement Difficulty Rating]
- Offline: 27 
- Online: 6 
- Approximate amount of time to 1000: 25-30 hours [If boosting Online achievements] [Estimated Time to 100%]
- Minimum number of playthroughs needed: 1 + partial [Highly recommended to do 2 full playthroughs]
- Missable achievements: Collectible and some miscellaneous achievements can be missed if playing on Extra Life mode, playing on normal mode none are missable
- Does difficulty affect achievements: No
- Unobtainable/glitched achievements: None
- Extra equipment needed: None
DOOM Eternal sees you take control again of the impossibly powerful DOOM Slayer and tasks you with tearing your way through Earth, Mars, Hell and anything fleshy that stands before you as you resume your eternal struggle against the Demonic forces of Hell!
The achievements in this game are largely straightforward. Though some online achievements are present, they can be boosted to cut down on time and perhaps a little frustration. You can choose to play and complete Extra Life mode first and get all collectibles and most miscellaneous achievements here, however you can miss some stuff if you're not careful since mission select is turned off in this mode, even after completion of the campaign. You can also not unlock the achievement "Master of Fasting" on Extra Life mode since it requires replaying a level with a cheat activated, though you can't replay levels on this mode and the cheat can only be found on chapter 10, so another partial playthrough would be needed to reach this cheat and then use it.
While this may seem the better option as opposed to two full playthroughs, there is a risk of missing collectibles and you need to finish Extra Life mode with more than 10 lives remaining, which itself is easy, however during your first time through the game, you may struggle and die a lot, not knowing what to do or where to go. There are also a lot of combat challenges and Slayer Gates that need to be completed that too can cost you a lot of lives, so this is why I would recommend a normal playthrough first to get everything done, then a second playthrough on Extra Life mode for it's achievement where you then would know the game and not need to waste lives with any of the combat challenges or Slayer Gates, which is how the Roadmap will be written - assuming two playthroughs will be done.
Normal Playthrough + mop up misc. achievements: Any difficulty
Your first playthrough should be done on the normal mode on any difficulty. I personally would suggest the easiest difficulty since it can be tough at the start until you get some upgrades, but if you're a veteran of the series or just fancy a challenge, feel free to do any difficulty you wish, it is irrelevant in regards to the achievements. Note though that there are 6 Slayer gates to complete and these can be challenging, though as with the rest of the game, they get easier with some upgrades and certain weapons. You can also return to any of them through mission select from the Fortress of DOOM at any time, though you need to replay the level up to the point where you can access them. Completing all 6 slayer gates will unlock "Bonus Stage" and "Breaker of Gates" and then using all 6 keys from these back at the Fortress of DOOM to acquire the Unmaykr will unlock "Meet Your Unmaykr".
It's the same story for the collectibles too; if you miss any during this playthrough you can go back to any mission from the Fortress of DOOM to find them and, conveniently when choosing your mission you can see which levels still have outstanding collectibles, so you don't need to keep track yourself. You can also see what collectibles are on each level by looking at the map and with some Praetor Suit upgrades, you can get even more details on these. There are also achievements for upgrading weapons and your suit as well as your health, armor or ammo. The means with which to upgrade these are all detailed in their descriptions but typically form part of the various collectibles or activities that are done on the missions. Finding all collectibles, including the means to upgrade your equipment and Extra Lives will unlock "Crystal Craving", "King of the Crystals", "This One's my Favorite", "They're ALL My Favorite", "Homemaykr", "Extra Extra Lives", "Reforged the Genie Lamp", "Metal Head", "Playset Sold Separately" and "If Only I Could Read…".
For time purposes I would suggest not leaving any mission until you have done everything on it, found all collectibles, completed the Slayer Gate, should there be one, etc.. At the end of each level before walking through the final portal/door a message will appear on-screen telling you that fast travel is now available and this lets you fast travel around the map so you can use this to collect anything you may have missed.
Other than the collectibles, there are some miscellaneous achievements, that will almost certainly come from just playing the game, such as "Darn It, They Keep BREAKING" which is for performing 33 unique glory kills and "It's a Magic Number" for killing 666 demons, among other things that will come with natural play.
The achievement "Treasure Hunter" is unlocked on the mission 'Taras Nabad' and is for completing everything on the level, all collectibles and combat challenges, so be aware of this when playing, though as with the rest of the game, you can choose this mission from the Fortress of DOOM to replay at any time.
After completing the game and all of the collectibles and miscellaneous achievements, you likely have only "Master of Fasting" left. You will need to replay a mission with a cheat activated for this, so see it's description for details.
Extra Life Mode Playthrough: Any difficulty (easiest recommended)
Your next step after completing the game and all of the collectibles and the like is to complete it again from the start on Extra Life mode. This game mode is separate from the normal game and is chosen from the main menu when starting a new game. I would suggest using a new save slot for this too so that you don't overwrite your previous playthrough, in case that is needed for any DLC. You can choose any difficulty for this and I would personally recommend playing on the easiest difficulty for the sake of the achievement "Running Up the High Score" which requires you to complete Extra Life mode with more than 10 lives left. This on the easiest difficulty is very easy, I had over 40 after my own playthrough so as long as you collect the extra lives dotted around the maps and don't die all that often, you will have no issue with this achievement.
Doing this on a second playthrough has the advantage of you knowing where to go and what to do and expect. You should also be much better at the game and the controlling of your character making this playthrough just a formality towards your completion.
Finally there is the online to contend with. You have two options here; play it properly and hope to make progress towards the achievements, though this will take a while and may not be to everybody's liking, or you can boost them all with two other people. The Online is an assymetrical fight with the DOOM Slayer versus two Demons. It feels largely unfair for the DOOM Slayer and favours the Demons and as such trying to unlock these achievements while playing the online properly may be frustrating and ultimately make the completion of this game much more difficult and take much longer, so I would personally recommend boosting them. For this you will need 2 other people and to play in a private match. Feel free to use the Official XBA Boosting Session Information & Discussion thread to find others to play with. Details on each of the achievements can be found in their descriptions below. The achievements unlocked online are "Mix and Match", "Fight Like Hell", "Blood Bath", "Man vs Monsters", "Weapons Expert" and "Truce between Demons".
Note that you can see your progress for each of the online achievements/requirements while in the Battlemode Lobby by pressing , then going to Milestones. Use to tab across to the Battlemode menu and all of your online progress for the achievements will be here.
So there you have it! An insane, brutally over the top Demon-slaying FPS. The single player campaign is fantastic and I'm sure you will enjoy every second spent trying to unlock the achievements. The online is hit and miss, some may love it, personally I wasn't a fan, but at least it can be boosted.