Vampire: The Masquerade - Swansong

MISSABLE

This trophy takes place in Scene 10 Part 3: Leysha – S.A.D. Base. This is a complicated and important level, because it's tied to 4 of Leysha's individual endings, as well as 4 different trophies (5 if including her death for the Worst Ending), so it's recommended to follow our Complete Walkthrough's Scene 10 Part 1 section for a detailed guide to seeing & getting everything you can in this Scene.

All endings you can get in this Scene & their trophies:

1. For the dead travel fast : Leysha managed to get to Halsey in time to save her. 
2. My flesh and blood : Leysha escaped from the SAD base with Halsey. 
3. Despair has its own calms : Leysha is locked up in the Vatican, unable to forget Halsey’s destruction.
4. From one prison to another : Leysha was unable to escape and is locked up in the SAD base.

Unfortunately, no matter what, you can't get all of them in the same playthrough, even using the cloud-save method. All of the listed trophies other than this one do not pop until you see the very last scene in the game, just before the credits, and all of them are mutually exclusive.

Right now, the only thing to do is to get the first 2 trophies (For the dead travel fast  and My flesh and blood ) as you work toward the Best Ending. 

Later on, during the clean-up phase, you can come back using Level Select to get the other two, both of which are for separate bad endings.

When you begin Scene 10 Part 3: Leysha – S.A.D. Base, you'll find Halsey in a locked incineration chamber, and you only have 10 minutes in which to do so. To get this trophy and For the dead travel fast , you'll need to save Halsey in time. 

Don't worry; 10 minutes is more than enough to do so and to explore everything. There's also a way you can pause the timer for several minutes, if you need to. (Grab a wrench from the red toolbox in a corner and use it on the cables behind Halsey's incinerator.)

The basic steps to save her (i.e. not including all of the extra things you can see & do in this Scene) are as follows:
First, head to the desk right in front of the incinerator. Open the top drawer and grab the Blue Keycard key item. Head to the console by the incinerator and use the keycard to open the glass covering the keypad. When you do that, as soon as you back out, you'll be immediately thrown into your first Confrontation with Halsey.

Confrontation with Halsey #1: (0 misses allowed)

1. Stage 1: Option 2: She knows it's me.

You'll have to figure out the code for the incinerator. If you want to solve this puzzle yourself, skip to the Halsey Confrontation #2. Otherwise...

Code puzzle solution: If you want to figure out the puzzle yourself: Behind a yellow screen in one of the corners of the room, you'll find a dead scientist. Interact with her body to take a Key from her. Go to the desk in the bottom-right corner of the room and use the key to unlock the top-right drawer. 

In it, there's a document that lists the codes for all the different models of incinerators. When you look at the back of Halsey's incinerator, you'll notice the beginnings of a model number, but the last bit is scratched out. You know it's one of the CLA 9000s, but which? If you look at all the other models around the room, you'll notice all but 1 model # is present: the CLA 9000L. (The correct code is: 260296.)

Punch in the numbers into the console, and you'll get the second Confrontation:

Confrontation with Halsey #2: (0 misses allowed)

1. Stage 1: Option 4: You need to trust Mommy, Juliette...

Finally, you just have to unlock the incinerator. Head back to the center desk that's in front of the incinerator and interact with the computer monitor. On the first tab, the Container Data tab, move your cursor into the screen and press Cross on "Open container". The incinerator will open and you'll have to beat one last Confrontation:

Confrontation with Halsey #3: (** misses allowed)

1. Stage 1: Option 1*: To get my little girl.
   • * Note: For Stage 1, if you got her teddy bear in Scene 07 – Red Salon, you can also choose: Option 2: To bring you Mr. Teddy.
2. Stage 2: Option 3: I'm here now.

** Note: The number of misses you're allowed here depends on how well you did in the previous 2 Confrontations. If you won both, you'll have 2 misses; if you won 1, you'll have 1 miss; and if you lost both, you'll have 0 misses.

Once you've won this last one, the Scene will end and you'll get this trophy. You won't get My flesh and blood  right away, but if you did the above and saved Halsey, you will get it when you see Leysha's final scene after the game ends.