i would like to pinpoint the comparison that you are making between Asura Strike and Final Strike.
• Yes, they are both 1-shot Kill in nature.
• Asura is melee while Final Strike is mid-range with 5-cell casting range.
•Asura and Final Strike are both Forced to neutral element.
•Asura deals a massive def-ignoring damage that can be increased/reduced by cards . Final strike deals a massive damage that does not ignore Def-calculation and can also be increased/reduced by cards. (This is the reason why there's a huge difference when it comes to Thana-user FS ninja)
Now, on the fourth bullet, Asura has this 'Incantation Samurai' effect.
We must start from there. FS ninja build with Incantation Samurai Card should be no more/less compared to Asura.
After balancing that out, let's take a look on how will it work with thana, and the changes should be made from thereon.
Please look into this.