I got this done grinding experience with Stark in Beat Attack (Beginner).
You have 3 mins to to take out as many Soul Reapers as possible, and they will continue spawning until the timer runs out.
I usually ended the round with between 250 and 300 enemies defeated. May not seem like a lot, but between the time of the round, leveling up, and loading screens you are looking at around 8 mins per round, giving you about 7 rounds and hour.
7 x 250 = 1,750 kills.
The added benefit of this method is, while using Stark you can reach some insane combos (I averaged about 1,500+ each round). This is great for accumulating Soul points as the higher the combo grants multipliers. For example, a 100 hit combo gives you x2 soul points, a 300 hit combo gives you x3, and a 1,000 hit combo gives you x4.
The higher the combo, plus the enemies dropping, will give you a nice soul point payout after each round (3,000+
). So if you are also looking for the trophy to max out a character, I would suggest this method with Stark.