What would the last human do in a world of demons

The demons appeared from beyond the gate to take over the world, starting a long war against a united human army. A war that humanity had no chance of winning. Gunpowder and bullets had no effect on the demons who used their magic to obliterate all who stood in their way, or so it was until the augmented humans were created. Powerful warriors with the ability to control gravity and fight on equal grounds with the demons. These are the memories of one augmented human who came back to life only to find his world ruled by demons.