Edit: Get rid of Dark Regeneration and use Soul Leech, it's far better.
Try to get 3-4 Embers before he spawns the felhunters, pop Dark Soul for this phase it will make a BIG difference and use Havoc on the Felhunters or you sure won't get them down fast enough
Also i might be blind but you are not buffing yourself with Dark Intent? Keep Immolate up! + use the Glyph for it (optional)