Learn your positional abilities well
As an assassin, there are no special attack abilities to use from stealth. Instead you need to make sure that you learn your positional abilities well, so that you can sneak up on your opponent from the right angle. Learn the limitations of the stealth abilities, Hide 1 and Hide 2. None of these can be used in combat, and Mortal gold can be seen by any class if they have Seeds of Destruction.