This achievement is similar to the Cyberdemon boss battle. You start with a worm crawling into a suit (so many pub jokes with that one!). this is the Hell Guard boss battle on the Necropolis level.
The first encounter involves his health bar being depleted, obviously!, first 50% of his health bar he charges at you and separately fires three energy bolts at you, this part is not too bad, bare in mind that he has an energy shield around him and you can only score damage when the shield is down, so when he charges and strikes you can dodge and score hits, I found the rocket launcher or the plasma rifle to be effective for this one and as suggested by toddycst the micro missiles on the HAR are very effective as well. The second of the attacks is the three energy bolts, the best way to deal with this is that you can shoot the energy bolts before he releases them and that stuns him (much like your own plasma rifle stun upgrade) while he's stunned lay into him, plasma rifle or micro missiles are best for quick fire and damage.
The second half of his health bar involves two different attacks, these are a little harder to avoid but not impossible, he uses fire this time. The first attack is lines of fire (apologies for not having a U Tube account so you can understand what I mean) when he fires this attack the fire rebounds off the arena wall at 90° so you need to run to the extreme of the arena in order to avoid it. the second attack is a strictly come dooming pirouette where rotating lines of fire will circle the arena, you can run in the gaps to avoid them, he will however, stop and start rotating the fire in the opposite direction so be prepared to run in the opposite direction. For both of these attacks his shield is down so give him hell! When his health is almost gone you need to run in and glory kill him.
After defeating the worm in the suit, like the Cyberdemon, he re-spawns, but this time there are two Hell Guards (same health bar, but it applies to both the Hell Guards i.e. you hit one it affects the other), this is a lot easier than it sounds, neither of the hell guards have shields or arena filling ranged attacks, so be patient and circle them, I used the double barreled shot gun for both of them but the combat shot gun with explosive shot works just the same, but hey if you have rockets, plasma or micro missiles (thanks toddycst) spare they get the job done quicker, so keep your distance and keep pumping rounds into them. When they get down to about 5% you need to run in and glory kill the first guard, after that keep hitting the second hell guard and he will go down soon after, rinse and repeat, run in and gory kill the second hell guard and job's done.
Additional strategy tips offered by Mickylove03
Micro missiles are the absolute best weapon for this boss. I had the upgrade that allows you to fire missiles constantly and this makes short work of both parts of this boss. Another good strategy is to use grenades. I chucked a grenade at the hell guard as soon as I saw him begin to charge up the triple orb attack , which not only stunned him & stopped the attack, it also did a good amount of damage.
Back peddling + circle strafing is also key to not taking damage here, and double jump those triple orbs if you mistime your grenade. Part 1 is easy.
Part 2 is easy also if you stick to micro missiles. The hell guards will drop ammo and health almost immediately if you start with the micro missiles, I didn't find this was the case with any other weapon. Switch to plasma gun when your ammo runs out, and switch back to micro missiles when you collect ammo. The best advice here is keep moving, and try and keep both guards in view and at a reasonable distance in order to dodge their attacks.
Please add to the guide if necessary rather than giving negative votes, the second half of the guide is a bit sketchy, I know, I was doing it from memory and it's the harder part of the battle, it's about helping not bitching!