Couldn't 6 levels of ranger do the trick ? As far as I can see the Sniper shot (from the cores) is like a 100% guaranteed bluff with a 6s cooldown ?*
Something among the lines of 8 rogue/6 ranger/6 monk halfling
Sneak attack dies
4 from rogue levels
4 from assassin tree
4 from ninja spy tree
3 from deepwood sniper tree
3 from halfling tree
========
18
Which adds what .. 63 dps per hit ?
*I guess the problem comes from - what to do after you kill one of them after a bluff and the rest are coming for you xD