It helps having a Merchants Guild, as they'll be trained right out of the gate with higher skill. But I find even with merchants trained in cities without, placing them on a resource for 5-6 turns earns them some skill and brings them up around 3-4 level. Then I'll move them to more lucrative resources, which pay better and will where they will (usually) not get bought out.

For assassins, kill every rebel captain you can within range until they've hit 3 or more skill, then send them to sabotage enemy or rebel settlements buildings. Once they hit 5+ skill and get a couple ancillaries, they have a good chance at just about anything you send them to do (assassinate, sabotage, etc.).