Two biggest things I learnt actually doing this fight myself (as opposed to just reading about it or watching vids):
1 - LOS is not needed or important to the fight AT ALL. It just unnecessarily complicates the fight. (Use demon portal debuff for CB and simple positioning for felhunters).
2 - Efficiency killing felhunters is key (IE Saving all CD for them, Having a few embers saved for them ready, using havoc correctly). I was new to warlock with my alt so I wasnt using Havoc correctly, which was all it took to make the phase trivial for me.