Mitsurugi Kamui Hikae

Just a tip for starters since it may not be immediately apparent:

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.

Otherwise, simply grind and save up SP to purchase all upgrades.
One of the required upgrades is the 100,000 SP costume.

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.

Okay so I know I have not gotten this yet but in order to unlock this achievement, I would grind it out on Easy to build your points...Now I thought that unlocking everything but the Costume would unlock it...it doesn't. For it to pop you have to "upgrade" the Costume and when I mean Upgrade I don't mean beat the game and unlock it...you have to spend the 100,000 points to get it.

Good Luck!