Mafia III: Definitive Edition

Getting only one Underboss is achieved by pissing off the other two enough for them to leave.
I chose to go with Vito, meaning I assigned all the districts to him. After he had gotten his fifth district, Burke left, and after the next one Cassandra left. Once an Underboss leaves you will get a mission to go kill them. I don't know whether or not this is actually required for the achievement, but it's in your best interest as you cannot use associates tied to the Underboss until you've taken care of them.

The achievement will pop once the last district has been taken over and assigned to your chosen Underboss for the achievement. It will pop after the last "Sit Down" has been completed (you don't get any choice in where the district is assigned, since you should only have one Underboss.)

Note: This is a missable achievement. If you are planning on getting all achievements, you will require a minimum of two playthroughs each with a different strategy. One strategy would keep one underboss alive (required for this achievement), kill all racket bosses, and kill 15 informants. The second strategy would keep all underbosses alive (causing you to miss this achievement), flip all racket bosses, and recruit 15 informants.

To get this achievement, you must kill two of the three underbosses. You do this by alienating underbosses by not assigning them districts and by not doing their optional side missions. Plan in advance. Decide whom you want to keep alive. Assign all the districts to the underboss who is to be kept alive. You may assign rackets to one of the two underbosses you intend to kill but, when it comes time to assign the district in which they are in, you must assign the district to the underboss you intend to keep alive.

Assigning a racket to an underboss you intend to kill and, later, the district it is in to the underboss you intend to keep alive results in the first underboss losing the racket and the second underboss gaining it. The first underboss reacts with great animosity hastening the rift you are trying to create for the achievement.

Story progression assigns three districts--Delray Hollow to Cassandra, River Row to Vito, and Pointe Verdun to Burke. The rackets in these districts are also assigned by story progression to the underboss of the district. The player is able to choose to whom to assign the six remaining districts. Assign those six districts to the underboss you intend to keep alive. The six districts the player assigns are Barclay Mills, Frisco Fields, Downtown, French Ward, Tickfaw Harbor, and Southdowns.

If the six assignable districts are given to the same underboss, eventually the other two underbosses will rebel and walk out of a district sit-down meeting. Make sure that you do not play the optional missions associated with the two underbosses you intend to kill because that improves their loyalty and could prevent you from unlocking the betrayal mission. Once an underboss walks out, he or she will begin retaliatory engagements against Lincoln and a 'Betrayal' mission will unlock. Bring up the map and select the betrayal mission. The objective marker will lead Lincoln to the underboss's stronghold. There, Lincoln must fight his way to the underboss and deliver the coup de grace similar to defeating one of Sal Marcano's lieutenants.

Once an underboss has been killed, Lincoln is provided with a follow-on mission to elicit the support of that underboss's lieutenant. The lieutenant will then continue to provide kickback and associate services. At that point, you may undertake the optional missions related to that operation.

Provided that you kill two underbosses, the achievement will be unlocked when the sixth assignable district (the ninth district) is given to the remaining underboss in the final sit-down session. This, also, is the event that triggers the Kill Sal Marcano mission.

I kept Vito alive because I felt that the advanced Consigliere's associate services were the most valuable. You save a tremendous amount of time if she is able to automatically deliver Lincoln's kickbacks to him. For simplicity's sake, I gave all rackets to Vito except for one that I gave to Burke. This was because I wanted to use C4 to kill the judge in the French Ward and to kill Frank Pagani in Tickfaw Harbor. Giving one racket to Burke unlocked Lincoln's use of C4.

I assigned a French Ward racket to Burke, killed the Judge with C4, and then did the Tickfaw Harbor missions before going after the French Ward capo, Lou Marcano. I concluded the Tickfaw Harbor missions by placing C4 on the pavement in the middle of the entrance into Frank's stronghold. I detonated it as Frank tried to escape quickly concluding the mission. This triggered the sit-down to assign the Tickfaw Harbor district. Upon giving it to Vito, Burke walked out removing C4 from Lincoln's explosives.

I recommend playing the betrayal missions as soon as they become available. Lincoln is harassed by the discontented former underboss with retaliatory attacks until he removes the threat. In my playthrough, Burke walked out after assigning four districts to Vito (in addition to River Row which is assigned to Vito as part of normal story progression). Cassandra walked out after assigning five districts to Vito. Vito was the only boss alive when the sixth district came up for assignment.

Note: After Lincoln gains control of the Delray Hollow, Pointe Verdun and River Row Districts and makes the mandatory assignments required by the game, there is some flexibility in how the remaining districts are taken over. However, lieutenants must be taken out before going after the corresponding capo. In other words, Barclay Mills must be completed before Frisco Fields to go after Olivia. Tickfaw Harbor must be taken out before Southdowns to go after Tommy. And Downtown must be completed before French Ward to go after Lou. Nevertheless, you do not have to do everything sequentially. You can start the prerequisites to go after one capo and switch to another. This is what I did when I killed the judge but did not, immediately, go after the capo, Lou. Instead, I switched over and started on Tickfaw Harbor which is a prerequisite for going after the capo, Tommy.

The below video is provided to clarify when the achievement is unlocked. It shows the final district being given to Vito, the only underboss at the table in the sit-down. In my playthrough, this occurs following the defeat of Tommy Marcano. The achievement unlocks following Vito's comments:


Note: This achievement and the one below are mutually exclusive and must be obtained in separate playthroughs:

This achievement requires that you remove 2 of the 3 Underbosses before you start the Mission "Yet Here We Are". It doesn't matter whether you choose Cassandra, Vito or Burke to be your final Underboss.
I chose VITO, so i will use him to explain my guide.
The game leads you through the rackets until each Underboss has control of one district each. After that when you gain control of a new district, you are moved to a plantation house where you must assign that District to whichever Underboss you want to control the whole city.(VITO),
To anger the other two Underbossess enough to make them leave, you need to assign one of them, eg Cassandra, the two rackets in a District, then, at the Meeting, assign the District to your chosen Underboss (Vito). This will make her angry, because when you assign the District to Vito, the Rackets are taken back and reassigned with the District. She will soon leave the meeting in a huff. You will then be given a contract to kill her.
After you get rid of Cassandra repeat the process for Burke. You must be finished by the time Vito has 6 districts.
Don't leave this too late in the game like i did or you will end up with 2 Underbosses instead of one and a 3rd playthrough to get the achievement.
Only Vito should be at the table with Lincoln when he assigns the final district.
Loyalty missions are not necessary if this is your second playthrough.