This is probably because I'm a beginner at this, but I don't understand why I'd pick an Assassin over the other classes.
Poison: nice idea but doesn't seem to give a huge advantage in practice against monsters high enough in level to be worthy prey. It's not like the Assassin gets super-fast healing from exploration. I suppose if APHEELSIK literally halted regeneration, then when paired with CYDSTEPP or BURNDAYRAZ it would lead to cheese.
First Strike if you explore around: very handy, but just feels like playing a less extreme version of the Rogue.
Swift Hands: Great, now I can butcher everything below me in level for no HP/mana/bad status... except I can do that to a large extent anyway with melee-oriented characters (or at least well enough that the few monsters who can't be slaughtered don't really bother me). Bosses are usually level 10 anyway. I suppose it is interesting though if you make your Assassin magic-focused to the point of gimping his melee damage, or if Dracul can somehow turn your level up to 11
Maybe there are also special dungeons with dangerous low-level monsters that I haven't got to yet.
So, how do you play an Assassin and make him awesome?