Then that should be included in the wording of the phantasm. 'If it lands correctly' is the words listed in the phantasm. If the effect of instant death can be resisted, and or dodged in some sort of manner so be i'll consider accepting it if no one else has an issue with it. Likewise however there are other assassins who don't use instantaneous death techniques such as Okada Izo's settlement sword, Sanson's doesn't guarantee death as one can pass a check to do so, Cleopatra's Uraeus Astrape (though her high skills make her character formidable in its own right.) and and Jekyll's ability to turn into hide. Not all assassins rely on an instant death technique. 

More so your characters stats are all very high that alone should render the technique not in need of pure instant murder. 

in comparison with Emiya Assassin, a counter guardian, for example 

[hider=E Assassin's stats]
D
C
A+
B
E (EX)
B++
[/hider]


or Danzou

[hider=Danzou's stats]

D[1]
D[1]
A[1]
C[1]
B[1]
C[1]
[/hider]