You should be able to get him now.
First, have you gotten Genoh? The big tortoise(sp?) looking guy at the Waterfall Basin. I think you need him first in order to get Byakuren.
Also, im not so sure if this is neccessary but you have to fight Byakuren twice too?. If its true then this is how mine went.
First time I fought her was after Nirva Island.
Second was after Queen's Campaign.