Magical Exemplar + the effects of a whole bunch of spellbook effects: was able to summon DMoC real easy and bring back monsters from the grave somehow. Honestly not even sure how. Gotta read up on the cards.
1x Dark Magician of chaos
1x Magical Exemplar
2x spellbook magician of prophecy
1x reaper of prophecy
3x fool of prophecy
2x spellbook of secrets (i assume 3 would be better)
1x chalice
1x spellbook library
1x spellbook of fate
2x spellbook of power
1x spellbook of eternity
2x spellbook of the master
1x treacherous trap hole
Magical Exemplar can special summon monsters from the grave too, so that's probably how. I want to say luck might be involved, but maybe he managed to thin his deck quick enough to draw both Magical Exemplar and DMoC. It doesn't look like it has any anti-backrow though. Something like WoD would probably mess them up easy.
I would have gone for Magician's Circle myself, which can be used to summon Reaper from deck, which can then in turn summon another thing (as long as you have 5+ books with different names in grave). I also expected Blizzard Princess or something, since her effect would actually shut down your backrow.