Get a solo lane, farm, max W first, grab a catalyst, philo stone or some doran's for mana regen. Wear down your lane opponent with Q > W and then Q > R > W > Ignite. In teamfights, Protect the carry or Q in as many as you can then R > W.
Buy stuff like, Rabadon's, Void Staff, Zhonya's, BV, Ancients possibly? AP per level runes? I have enough damage. But getting a solo lane is your best bet, Orianna farms very fast. Get a level advantage?