I'm not sure how you expect doomsayers to work in this deck.
Doomsayer is only there to stop early aggression which this deck is going to get crushed by. There are no other AOEs in this deck.
Besides it was really tough filling up the slots with all Epics.
I can explain the logic behind all the cards:
Giants: Pretty obvious why they are in there, this deck plays similar to Handlock. Sea Giants are in there because you will get swarmed by minions and this can allow you to put a big body in early (not that it will help).
Enhanco Mechano : Only way an all epic Warlock deck can get taunts in. Severely unreliable but it can combo with Blood Knights as well.
Pit Lords: Basically your only solid 4 drop minion. Synergy with Demonheart.
MCT, Shade, Blood Knight: Usable 3 drops. In certain situations can result in huge swing. Bloodknight has synergy with Enhanco.
BGH: Big removal.
Faceless: Faceless your Giants or your Pitlord with Demonheart. Or their taunts.
Recombobulators: Mostly to heal your Giants or convert a couple of them into Deathwing.
Bane of Doom: Small removal that can hopefully get you something huge.
Demonheart: Another much needed removal for this deck.