Lulu is a really long range poke harass champion. She should be able to fight, all the way in the back lines, at all times. With the use of Help Pixie, she can use glitter lance at around 1000 Range. Because of this, you can build her without any defense. At the same time she has a shield and a non-ignitable heal so you can also build her as a tanky AP mid if the situation calls for it.
The most important thing about Lulu, is that she can win exchanges versus most champions mid and can usually deal unreturnable damage. Using help Pixie she can deal damage with glitter lance well out of the enemy's cast range. However, when you do decide to exchange attack damage, her passive will grant extra damage making you come out ahead against most enemies.