Across several hero thieves, my kill/death ratio against assassins is around 10:1. That is better than those characters' overall kill/death ratio against all opponents. Looking at those same characters, the only classes where my kill/death ratio was less than "1.0" were anti-paladins and necromancers.
The very strongest assassins were a struggle to kill, but that's true for the "very best" examples of other classes as well. The strongest assassin is generally less of a threat to me, as a thief, than the stronges warrior, invoker, berserker, anti-paladin, transmuter, thief, etc.