Zantetsuken on NMs seems to be based on the mob's HP, instead of the Summoner's MP.
I'm not sure if it's total HP or remaining HP but if it's an NM that already has a naturally low HP pool the numbers will be disappointing.
I've been playing around with it in besieged for a bit and saw completely random numbers going from 43 on a whm to 1999 on a NM.
And one of mine outside besieged:
(Although I should have waited for when we popped Yilbegan.)
The theory of it doing damage proportional to the mob's HP comes from here though:
(Screenshot is NOT mine, courtesy to BG)
And another one by Spira on KI:
I also found that the damage of Zantetsuken is divided when used on a number of NMs, but I have yet to confirm.
Accuracy when used on more then one mob seems to be terrible, it's that I've personally occasionally seen it hit multiple targets, or one could have thought this pact is single target.
What the exact modifications for that are is something still to figure out.
[Edit]
Damage does indeed also seem to be influenced by the summoner's mp pool.
Just need to find out how much, according to numbers I've seen it is quite considerable.