The best combo I have found is Carrion Feeder, Pawn of Ulamog, Zulaport Cutthroat and Blood Artist, 2 or more guys that you pay to bring back the best being Reassembling Skeleton and if course a Grave Pact. With 6 lands.
Use Carrion to sac the 2 skeletons. That activates zulaport and artist so opponent loses 4 life and you gain 4. They then sac 2 creatures from gravepact , which then activates artist. They lose 2 more and you gain 2. With you sacing the skeletons ulamog triggers giving you 2 eldrazi. Proceed to bring your skeletons back by using 2 mana then sacing your eldrazi for 2 mana. They then sac 2 more due to gravepact. They lose 6 life you gain 6 life from sacing them. Repeat 2 more times until out of mana. They will have lost 12 life each skeleton sac. So with 6 mana you can do it 3 times which causes 36 life loss by turn 6. You can usually pull it off sooner.
Damn, dude! This is harsh! I'd suggest Butcher of Malakir, but he completely breaks your awesome low cmc mold. I know first hand how much value pawn of ulamog offers. Good pick! If you wanna, you could have a gander at Sanguine Deluge. I run a very similar concept with the lil' suckers