Ignite + Flash to quickly assassinate caught foes. This works best accompanied by a CC lane like Taric or Leona.
21 In offense to deal the most damage while having 9 in utility to cushion your heavy mana consumption. Furthermore, having 9 in utility helps you clear the wave without worrying about your mana, but this doesn't mean that you should always spam your skill since this may leave room for the enemy to strike. You could substitute 9 in defense if against a poke lane.