Just Cause 4

To earn this achievement, you must complete the final mission in the Los Demonios DLC. This mission leads to a boss battle that takes place in three increasingly frustrating phases. I won't go over what you need to do to reach the boss, as it's not necessary. I just want to share my experience with the boss battle itself. Hopefully it helps some people out.

The first phase is easy enough as you'll just be doing what you've been doing to take out the sources up to this point. Pull open the flaps with your tethers and blast the weakpoints. You'll get a checkpoint after this phase.

The second phase can be annoying. Scan for weapon caches (there are a lot of them in the area) and pick up rocket launchers and/or railguns. The loadouts in these caches seem random, but you should quickly find what you need. You'll need to take out 8 "infested weapons" (tanks, etc.) that are attached to tentacles. Rocket launchers and fully-charged railguns destroy these vehicles quickly. You'll have to do all 8 in one go (no checkpoints mid-phase), so move quickly using the tried-and-true parachute/wingsuit combo or else you might die quickly and randomly if you stay still for more than a few seconds. There were several times where I seemed to die in one hit from who-knows-what. You'll get another checkpoint after this phase.

The final phase of the boss is very frustrating. If you don't already have a rocket launcher and a railgun with full ammo, I'd recommend checking the weapon caches until you have this loadout. Use the rocket launcher to take out all of the infested vehicles that are captured in the tentacles around the boss before doing anything else. Maybe the boss can be killed without the vehicles being destroyed first, but if so, I imagine you'd have to be lucky. They constantly knocked me around and killed me before I got even a few shots off on the boss. After destroying the vehicles, use a fully-charged railgun shot on the boss whenever possible. Even with the vehicles destroyed, you'll still be contending with demons, so keep on the move and focus on killing the boss.

Update: The SCHWARTZ 00 suggests in the comments to use the PBX Mine Launcher on the boss in the third phase as it drains its health quickly.

Once you take out this third phase, the mission will end and you'll get your well-deserved achievement.