Merchants and Assassins

[FactionHeir]

Always train merchants in a settlement with at least a town hall, that gives them the LegalDealer trait.
Then move your merchants around expending all movement points every turn without stopping on a resource. This you can do with an aim of reaching far away resources which tend to be more profitable. They get the WorldlyMerchant trait that way. This already allows a total of +3 finance and they won't get attacked this way.
Once you settle down on a resource, keep them there without enemy merchants in the same area as long as possible to get the Monopolist line of traits.
Use them to acquire enemy merchants only if you at least have a 60% chance of doing so.

As for assassins, arson is good. Once they get GoodConspirator up to 3rd level, use them to target skill 0 diplomats and captains with small regiments. Parking them in settlements with high level brothels also gives them useful ancillaries