In addition to previous solution, i wanna to say that if you have A+ in all subcategories except damage (for example b+) and you have 45000 in finalization points.
You can do this: start mission 1 on hard, in the first fight against the Pox giant, when is ready for todomé, start to hit him a lot with dual Knives (LT+X combo). Let him recover life but you can't stop hitting him, when giant is ready again for todomé, press ninja vision to not have cancelled your hit combo because you know the action stops a lil bit when bosses are ready for todomé.
After that continue your hit combo and cancel ninja vision, rinse and repeat... let him recover life, smash button like a mad with dual knives, when todomé is coming press ninja vision...
I reach 379 hit combo and finalization points go from 16000 to 33000 on mission 1. I unlocked achievement by this way with 52000 points overall A+ in everything except damage(B+) because i don't have last ninjutsu.
EDIT: Thanks to Nightrain89
i missed to say what movement i used to do high combo, i used LT+X. Sorry :p
EDIT2: As some users said, it seems that you can do this method without having Dual Knives at level 5. Thanks to Shinobi of Edo
, he said this and could be helpful too: "Using Ninja Vision during the first Pox Giant fight proved to be quite tricky and didn't work so well. I would often get around a 200 plus combo and have it reset on me. I found that using the maxed out Plasma Shield ability (obtained by finishing the game on hard difficulty) worked great! After striking him with the dual knives for a while, once the Todome moment comes, use the Plasma Shield instead of Ninja Vision and it will continue to hit the Pox Giant even when the action pauses. Doing this, I was able to very easily get a 999 hit combo in just a few minutes, pushing my overall score from 44000 to 64000, WAY over the score needed for an overall A+ ranking!"
EDIT3: Thanks to Kjer25 "if you are going for all the other achievements and replaying levels you can get this achievement through natural progression and without beat the game on hard. Going for the one weapon and no deaths achievement should grant you this achievement through natural gameplay."
Thanks for your help Kjer25