In this video guide I will show you how to defeat Dagon, one of the Lords of Hell. Now I think this is the worst boss fight in the game. What you want to do is focus on his minions first during the whole fight. If you don't you will get overwhelmed.
After a bit he will throw tidal waves at you, so hop on the air lift from when you came in on. After he does that he will also summon water orbs that shoot at you. Take these out right away as well. At some point he will also have a pool of swirling water at your feet which will hurt you, so make sure you know where to stand. The last move he has will be calling down lightning strikes on your head.
So with all that going on just make sure to take out all the minions you can and dodge as best you can. Try to build your ultimate up and use that as much as you can until he is dead. The achievement will pop after the cut scene.