In this video guide I will show you how to defeat Belial, another one of the Lords of Hell. This fight really is not too bad. Just use Strife to get him down to about a quarter of the way. At this point Belial will start to suck in minions for health. It's best to stay on the bottom of the arena and use Strife to shoot the minions as they get pulled in.
This will be one thing to watch out for.
The other part of the fight would be not too long after the first part I mentioned. He will start having the four crystals in the corners light up. You will notice them when there is a beam connecting the crystal to Belial. To turn it off use War's gleave to throw at it.
The rest of the fight will be these few things but a little bit harder. Just make sure to keep dodging and shooting to bring him down. Pay attention to his moves and stopping him from gaining the advantage with his health regain and buffs. Once the cut scene has ended the achievement will pop.