Just go to the battle square and win enough BP. You need A LOT of BP to buy Omnislash on disc 1 though. It costs 32,000 BP on disc 2/3, but it's a lot higher on disc 1.. can't remember the exact price, but it's definitely above 50,000, maybe above 60,000. You need patience above all else to get it, unless you have severly over-leveled characters
