Well your dmg is armor ignoring if nothing else, but i didnt go dmg in late game, since i capped RLord n wanted to try it.
Dropping baddies? You can beat Shiro with henchies. Lemme copy paste:
Beating Shiro with 1-man-army Ritualist
You run up near Shiro, and stop just before aggro range (after cutscene). Then:
I'm using Pain!
I'm using Bloodsong!
I'm using Wanderlust!
I'm using Shadowsong!
I'm using Dissonance!
Run up n smack him with wand, send henchies on attack and:
I'm using Painful Bond!
I'm using Celestial Skill!
...when Painful Bond recharges, re-cast it.
GG.
-------
The 8th skill is Charm Animal, but if you dont need Phoenix, you can take something else

It should be noted that i didnt even use Celestial Sigil first time i got masters on this.. i didnt want to ruin my skillbar i had then. But that was with a PUG.
As for henchies, no eles. Im not expert in this mission, but doesnt Shiro have a skill which heals depending on the highest dmg he received in some period of time? So, technically, causing a bunch of small dmg might be better (thus, spirits are perfect).
In any case, Ritualist reigns supreme once again.. artillery ftw
