Let's see if I can recall my setup on Belial!
Hungering Arrow (Devouring Arrow), Elemental Arrow (Nether Tentacles), Smoke Screen (Lingering Fog), Preparation (Backup Plan), Caltops (Torturous Ground), Marked for Death (Mortal Enemy), Steady Aim, Archery, Sharpshooter.
In P1: I used Caltrops to sort of cut off the left side (With two placed, one above the other) and ran to the right hand corner, where I spammed Nether Tentacles to my hearts content.
P2: Again, I kept two Caltops down (replacing them when Belial dashed away or w/e) to root random snakes, popped a MfD onto Belial, spammed Nether Tentacles on him, while vanishing snakes and dodging his projectiles. (Though P2 was by far the hardest phase to do, imho).
P3: Just dodge stuff and pewpew Nether Tentacles, again I was at the right hand side of the platform as I believe the breath can not hit you there.
You could also consider dropping Hungering Arrow and picking up Bat Companion. Bat, Mark and spam Tentacles to generate Hatred, but I found it a bit meh on Belial.