I was going to post a comment about using tarts in besieged fortresses as assassins.
But this got me thinking about skratti. This may already be on the bug list (and even if it isn't considered a bug, it is an exploit) or already be well known, but a skratti with a black heart can sneak into a province, the next turn change shape into the giant form and issue the assassinate command, using the script change shape + attack/cast spells. After the assassination attempt the skratti will change back into wolf form and still be hidden.