Call of Duty: Black Ops 4

This achievement is for the Specialist HQ mode. In order to get this achievement, you must do the tutorial of every operator which will earn you the MP 101 achievement (Note: the achievement for MP 101 is currently bugged). After you complete the tutorial for all of the specialist then you must get 3 stars on all the skirmishes along with doing miscellaneous challenges pertaining to this mode. To see what challenges you need you can access it in 2 ways. You can either hit when you're at the specialist HQ main menu or you can click on story then hit to go to the intel tab.

Here is a list of all the challenges and how to complete them (Note: All the challenges do track your progress).
The Black Hole: Earn 10 bronze stars. Simply do the first skirmish of all the operators which will earn you the Stargazer achievement.
The Immortal Candidate: Get 15 consecutive kills in 1 round without dying. This one will come easily since the bots aren't that intelligent so just use whatever class you like and use killstreaks to make it easy.
West of Eden: Get 10 kills with each of the primary weapon classes. You can choose classes mid skirmish and the classes you need to use are Breach for it's SMG (Corodite), Mercenary for his assault rifle (KN-57), Support for his LMG (titan), Sentinel for his Tactical Rifle (Swordfish) and Marksman for his sniper (Koshka) you do not have to do the Hunter class as a heads up. Note: If you happen to pick up an enemies weapon of a type you haven't done yet it does count towards the challenge.
Raul's Way: Get 25 melee kills. Simple enough just go around and melee the bots half the time they won't notice you're doing anything to them til it's too late because they're staring at a corner.
The Running Woman: Get 50 kills with secondary weapons. The only classes that have secondary weapons are Mercenary for his pistol, Support for his rocket (wouldn't recommend for farming), Sentinel for his SG12 shotgun, Marksman for his Spitfire smg and Hunter for his MOG 12 shotgun
The Panama Suspect: Earn 10 silver stars. Complete the 2nd skirmish of every character and this will unlock.
Kiss of Death: Get 100 headshot kills. Simple enough just use any weapon of your choice and just aim for the head you'll know it's a headshot if it has a different icon it'll look like a crosshair with a skull in the middle instead of your usual icon when you kill someone with whatever gun you're using.
Dead Again: Get 50 kills using scorestreaks. You'll most likely earn this in your time of getting all the stars but for my setup, I used UAV, Hellstorm missiles and the regular attack helicopter.
Eternal Sunshine: Get top of the scoreboard 25 times. You'll most likely earn this when trying to get all your stars since the AI teammates don't really get that many kills or play the objective well in a few game modes.
Lost Highway: Get 100 kills using the specialist weapons or equipment. Firebreak makes this challenge extremely easy with both his purifier 1 shotting people and his reactor core taking out enemies like nothing along with the added bonus of taking less damage when you use it.
Mason's Ladder: Get 150 kills on veteran. Veteran is the difficulty they use for the 3rd skirmish on every specialist. You'll most likely earn this while trying to get 3 stars on all the specialists. If you haven't finished it Hardpoint and Domination mode is the easiest to get kills on.
From Borkuta with WTF: Earn 10 gold stars. Simply complete all 3 skirmishes on all 10 specialists and you'll finish this challenge and earn the Centennial achievement.

Once you complete all the challenges you'll have every video and audio files in the game to unlock the achievement. Now that you have the achievement you can now listen to all the audio files to get more of the backstory of all the specialists if you choose to listen to them.

I think the text guides here already cover everything you need to know. I just wanted to offer a video guide as well. I hope this helps. This video will show you how to unlock all 5 achievements in the specialist HQ. We will be unlocking the following achievements: MP 101. Stargazer. Centennial. Skirmisher & Sleuth.