I personally found this and the 2,000 slash combo to be near impossible when I first started, though with some leveling it can be much easier. First and foremost you need to level your character of choice to do this (mine was Ichigo) up to obtain the extended combo time nodes. These help tremendously as you are progressing through the stage as it near doubles the time. You may need to level other characters to reach different parts of the development map for these. These grids will be the ones you want for combo enhancement.
Soi Fon Subgrid
After obtaining these, I suggest you obtain more bars for your spiritual pressure if you're going to use a similar method to mine. Personally I used standard Ichigo for this. The combo you'll want to use is square x 4 and then jump with X leading in to the Tenburenjin (used by pressing circle rapidly in the air) which builds up the combo nicely. If you can launch more people in to the air, the greater the combo will grow. It's also worth noting if you are in an enhanced state the move will pick nearby enemies up off of the ground, making it easier to chain them together. I'm sure you could find ways to do this with other characters as well, but I simply found this easier.
If you'd like a suggestion as far as a stage I personally did this on Mission 3, very hard difficulty. The enemies are resilient so you have plenty of time to keep them in the combo and the boss at the end of the stage is Rudbornn and he supplies you with infinite enemies as long as you don't kill him. If you kept the combo throughout the stage, you shouldn't need him to spawn too many. Watch the 8 minute timer, as this will end the level. Dashing into enemies helps keep the combo alive even if the enemy is guarding, this is a helpful tip to keep in mind. Also, dashing in to the nearby item pots will help your cause tremendously while he drops more enemies for you to combo.
This link below helped me when trying to find the best means to level.http://www.gamefaqs.com/ps3/611857-bleach-soul-resurreccion/...