Boss Fight: Tremonius
You’ll need to place the power seed in the crucible to restore power to the elevator, then you can deploy the Weapon to the terminal to activate it. However, before you do this we’d recommend that you stock up on any grenades in the area, and make sure you have a weapon with a decent amount of ammunition in it. Ideally, you should look for a Pulse Carbine, since this will be most useful for the fight ahead.
With the elevator activated, an elite Brute named Tremonius will come down and exit it with some Jackals. These Jackals have large orange shields which are more powerful than their blue equivalents. They also have more armor and inflict slightly more damage, so you should focus on taking these out quickly to eliminate the risk of being shot from behind or from the side. This is where the Pulse Carbine comes in, since it’s extremely effective at destroying Jackal shields in just a few hits. If you need to finish the Jackals off, throw a few grenades.
Once the Jackals are dead, turn your attention to Tremonius and fire at him to deplete his energy shield. You won’t be able to damage his core health until you do this, so it’s important that you maintain fire on him to ensure that the shield doesn’t have time to regenerate. Even when the shield is fully depleted, it will regenerate to full again if you don’t damage Tremonius for a while. the Pulse Carbine is again the most effective weapon available to you for eating through shields, so try to use this at first. You’ll probably run out of ammunition with the Pulse Carbine before you kill Tremonius, so you’ll need to finish him off with grenades, other weapons picked up from the area, or with your pistol.
Once you’ve defeated Tremonius you’ll unlock the Ascension achievement. All that remains now is to use the elevator to reach the surface, thereby completing the mission.