I play an assassin, and i get your point.

Assassins die VERY fast, but, they kill with equal speed (If not faster).
You see, the thing is, the ussual combat for me is like:

I Dash in, (To give casters less time) then throw out a combo of up to 5 different attacs, the enemy dies.. And this is were i run out of energy..

THATS when assassins die. When they cant kill.. So i guess they are most suited at taking down lone targets!

I do see, it must be a frustration for monks, but you could ask for the assassins in your group to take some deffensive skills (If not to much against their build) In particullar, ask for : Deaths Charge (Health+teleport) And Shadow refuge (+5 regen for a short amount of time)

Hope that helps..