I've actually acquired and upgraded all the celestial weapons. The butterfly game and chocobo race were easily the hardest to get, but I did eventually manage to get them all. Auron's rocks. Triple Overdrive, Initiative, and Break Damage Limit combine to let him instantly kill almost all random encounters. Set him in Slayer mode, and trigger his final Overdrive in any fight. Opponents die, and his overdrive gauge will fill up immediately in any group of 3 enemies.