It's a combination of several things. My accessories all buff final blow by 8% each. I reforged my sword for final blow buff. I have a maxed out kusarigama skill with like + 8% final blow damage.
I'm full Iga Jonin so I get agility damage buff. All my stats are pumped into heart and dex (43 each in)
I also pop medicine pills and hit honda with weakness talismans.
It's also great for parrying. In my first attempts I was just parrying and hazing Tachibana, but I'd eventually miss a few. With the parry you get an extra backstab hit.
I actually like my damage from behind more.