As to merchant, everything has already been said...

As to assassins, I'm big employer of those (my faction leader almost always get named the Killer, the Malevolent or the Tyrant....).
The main thing beside training them with sabotage missions is finding a princess...often they have no charm whatsoever or only one level and even a low level assassin get favorable odds against her. Moreover, you'll get the skilled courtesan ancillary which gives a +2 lvl.
I generally train my assassin on each and every captain within reach as even with a one eye assassin you'll get a + 1 lvl for each kill until third or four thus raising it to level 3 or 4. Add sabotage mission and retinue and you can get to a +7 or +8...
It needs of bit of micromanagement but it may be worth it...

I generally make use of high level assassin (My best reached lvl 13) to get rid of a faction in one turn
Use a stack or assassin to kill leader or heir and same on any other family member and bye bye faction, hello rebels...