Mitsurugi Kamui Hikae

Essentially the same solution as:


Once you've beaten the game, you can immediately restart or go to your save file and you have two options "checkpoint" and "game restart". If you restart in this manner you can keep all your SP and upgrades from the previous run.

So this does not need to be done in one play through.

You'll need to save up 100,000 SP at once anyway for the required costume upgrade.

SP is gained based on your zeal, you'll notice the red orbs grow larger as your chain grows. Your level of zeal is noted by the wings/crystals(?) to the left of your health/katana bars which turn from silver to red as your zeal grows. Being hit causes you to lose your zeal streak. Using your Zanshin () seems to yield more SP when charged with red rings and does not reset your streak. Ultimately, it is simply a matter of time.