You will attain this via standard play. The Solution is more of a tip for the Final Boss.
*** Spoiler - click to reveal ***The Hydra.
Act 3 Chapter 8: There are 2 parts to the mission.
Part 1.
The final level begins with your 4 units together in a circular like room.
From the start you will have to move around the map while hitting 3 switches. There will be enemies. (Wretches and drones) and 1 Boomer. (apologies if I forget a type)
As you take out the switches. You will see some doors open and a couple drones come out.. (Keep this in mind as this will be an enemy spawn for part 2 as well.)
Once all this is done and the 3rd switch is hit. A cut scene takes place. The Final boss has now appeared.
Part 2.
With the boss out in the open. Your squad is now divided in teams of 2 on both sides of the map. Similar to the Brumak Boss from Act 1.
The Hydra has a rocket barrage. that you can step out of the way. You can try shooting the guns on its side. (However unlike the Brumak fight this does not really change much? I could be wrong but unsure. The guns on its side would simply get fixed and made the fight longer than it needed to be. If someone could confirm if shooting the missiles impacts the fight as for me I was nearing over an hour and like stated i noticed it was simply making the fight long.)
When the missiles shoot the Hydra will swap sides. I had Gabe and Reina together. With Sid And Mikayla on the other.
The doors from earlier will open and locust will spawn. 2 Drones, ticker, and a wretch will spawn on both sides.you can try to lure them to where the rocket Barrage will land and let the Hydra try to take care of it. You can try to plant a nade at the doors and it should help slightly with spawns.
As the Hydra gets closer to death. The spawns may switch to one side getting a boomer. you can try to use it for crowd control should you be getting overwhelmed. Or you can try to shoot it at the Hydra itself (the damage was pretty decent.)
The Hydra was a pretty long fight. Try to keep your units a bit spaced from each other. For Reina if the Shotty does not reach switch to the Snub. Use your abilities/executions when you can. Having the Sim healing and empower on Gabe as well as perhaps having one or 2 units bring the Sim Nade since frags may not be as useful as having an extra healing aid isn't so bad.