What type of an assassin is it? Im assuming it's a martial arts assassin.
The gear you've listed he's wearing isn't all that good. I would advise lvling him a little more, then, assuming u can afford it, grab yourself an :
SS
Shaft
Gaze
Crafted 20ias, LL, +2 Ma Skills Gloves
Swap out a Raven frost for a High LL Ring
possibly some Perf Waterwalks
Sadly, it's all about the gear. If you have crappy equip, no matter how skilled you are, you just wont be all that good.
Also, post you stat and skill point allocation, ie:
str, vit, dex, energy (better be base), and what skills you maxed