Some tips on how to deal with it.
The first stage is fairly easy, you just need to avoid the rockets and airstrikes. Save the BFG ammo for the second stage since is much harder than the first one. Once the health is depleted, don`t forget to go to boss and perform a glory kill, otherwise the health will replenish back again.
Second stage is made up of three mini-stages:
First one is where the boss will raise some stone walls and either shoot rockets at you or energy waves. It`s worth to notice that you can escape the rock walls by strafing to the left or right, seconds after he raise them. Also the walls feature some sort of openings from which you can double jump outside of them. To avoid rockets all you need is to run or strafe continuously either left or right, doing so you will not get it by them. This also applies to energy waves.
Second mini-stage: You will notice it triggering when the boss will drop a bit of ammo. Besides the above attacks, boss will try to fire it`s laser at you; laser that is one hit kill. So be wary of it.
Third mini-stage is the most dangerous one since it will fire both energy waves at you and air-strikes. I recommend to use BFG in this stage because it will stagger the boss for a few seconds, giving you health some ammo and the chance to finish the boss off.
- The only thing that drops from boss besides some ammo at the second mini-stage is 1 BFG ammo.
- If you saved some (BFG ammo) this stage will be much more easily for you.
- For the first 2 mini-stages you can use shotgun explosives shots, mini-gun missiles or even the chaingun or gauss cannon. But shotgun explosive shots and grenades, do a decent job at taking the health down.
- For the last mini-stage after you stagger it with the BFG, shoot it with the rocket-launcer from a distance or you could try to super-shotgun from up close.
- Besides the health that drops from staggering the boss with BFG, there`s no other way to regain health, so be super careful about it and try to preserve it.
- After you take down all the health, you need again to perform a final Glory Kill on the boss, which ends the boss fight, getting you the achivement.