Dark Nightmare / GY Sac - Soul Eater & Monstrosity
This was my attempt to put the Shaytan Monstrosity somewhere, first I tried with Oaklings but didn't work for me so here we go:
Most of you already know how to play a gy sac deck so the start it's obvius. The point to be able of enjoy the Shaytan Monstrosity is to play it when one of your buffed Bone Collectors/Soul Eaters/Shaytan Monstrosity(OTK) or your Oblivion Riders/Death Walkers will die, this deck has creatures with nice damage and some buffs, you will have your chance.
Try to play the Oath to Oblivion on the Elderwood Hermit, I don't know why but most of the time the Rider gets the Hermit's buff :v.
The Shaytan Monstrosity has low health so keep it far from the oponent's creatures, it is a finisher, be smart with it. Spirit of Rebirth and Erodon can help you with this low health issue.
PD1: Put another Spirit of Rebirth if you don't like Doomgate.
PD2: When a Shaytan Monstrosity dies absorbs its own damage, so if you have 2 in play when one die the other will get a BIG damage buff.
PD3: Sorry for my bad English.