Void Sword took a while for me to get as well. There are three ways to get it: use Asbel's a-artes 12,000 times (all of them combined), kill an enemy with a 50-hit combo 5 times with Asbel (which is what I did), or sell an armor piece that is worth over 50k.
It was Asbel's last B-arte that took me the longest time. Didn't get it until he hit level 100.
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Congrats to 2011 Rivalry Rumble Guru winner, SuperNiceDog
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