I'd imagine the main thing for such a build isnt max Helpless Damage, but maxing your uptime and DC for Helpless itself. I dont know how long you'll be able to rely on GCS to consistently apply Helplessness.
Dlord Roar is probably the most reliable to build around...can get your DC pumped pretty high, it works on everything, and its got a decently short CD. Improved Trip with T5 Dreadnought and THF would also be a good way to apply it steadily, though there's plenty of Trip immune mobs to worry about.
Another option is to use a Guardbreaking item...that will give you one round of Helpless damage after every crit. Maw is probably the best way there, since Kukri has a big crit range and can be centered for use with Ninja, and is generally a good weapon on its own.
Also dont forget Horc gets 20% Helpless damage in its racials. But each of the No Mercy line is a T4, meaning each one will take 23 AP. That only leaves you 11 AP, which is your GCS, and then that's it. No T5 enhancements thus no crit bonus, no WIS trance, nothing from Henshin or Shintao so very little MP. TA and Ninja are pretty weak trees on their own merit, especially with no T5s.