Belial
The Lesser Evil of Lies finally shows his demonic self and throws everything he has at you. This fight is long and likely the most difficult you’ve encountered thus far. It’s broken up into three parts so let’s start with the first.
Upon entering you will face his personal henchmen. They are the snakemen you have been dealing with throughout the second act and not very strong. As long as you don’t let them mob you, you’ll be fine. After they are all gone part two begins.
Now Belial jumps into the fight and starts summoning more henchmen. At this stage he will typically stay at a distance and sling some magic. The green energy balls are quite powerful and you would do well to avoid them. He can also summon a blue energy field around your feet. This isn't that bad and you can simply walk out of the AoE to stop yourself from taking damage.
The final stage is the most difficult by far. Belial will transform the battlefield and show his true, giant form. His attacks also change and he’ll be hitting you for much more damage. His main attack is a simple melee swipe that can take out a good chunk of your health. You do have time to dodge it as a green circle will precede where the attack will land. He will follow it up with a few more attacks and if they all hit you, you’ll likely die.
Belial has another major attack that covers the battlefield with explosive black and green circles. He will stick his arms into the ground to start spawning the circles. After they appear they will explode about 3 seconds later, so make sure you stay running and on clear ground.
His final attack is a simple breath that sprays across the area. It does little damage and even if you get hit you won't have much to worry about. Characters with the ability to move quickly or mitigate damage will be particularly useful in this fight. The best example is the Wizard and his Diamond Skin. The ability to totally block the incoming damage is great for tanking through hits to let you continue damaging him, or to revive allies.
Ghom
The Lord of Gluttony infects this dark chamber and it is up to you to remove him. Fortunately he is a rather slow demon and this fight should not give you much trouble. He has three main attacks. His melee attack isn't very strong, though it is often paired with a vomit move that adds on more damage. If you don't have much health you should try to avoid this, otherwise you likely can just tank through it.
His second attack leaves a trail of toxic gas when he moves. It can do a lot of damage if you stand in it for long, so make sure not to linger in the green fog.
His final attack is the ability to summon Acid Slime minions. These minions are actually very powerful and you should try to stay away from them – as you can see in the video (they killed our Demon Hunter). The slimes move pretty slowly so avoiding them is easy and they have very little health.
Ranged characters will have an easier time during this fight with their ability to dodge attacks and stay away from the cloud of gas. However, he is fairly weak character overall and with a steady stream of incoming damage, he won't last long.
