well to keep it simple me and eternal inferno did this in a "player match",we took turns at winning 10 matches each,doesn't really matter you could take turns after each match if you want,
set the matches to 1 round,hit the start button when the match is starting to skip the scenes and just continue on to the next match
i didnt time it but i think 1 hr should cover it,our matches took about 6 sec.
use nightmare and ring out your opponent with a (Y) hit while moving in on him with the left analog stick,(be sure you pick a area where you can ring out like UNKOWN FOREST level. hope this helps
this will also get you the HANDS OF THE ABYSS cheevo...