Lulu's damage is insane. You can still support your team all game while going for a straight AP build. I usually use AP straight up for Lulu in every game.

Your basic strategy if you mid AP Lulu is to poke with Glitterlance [Q] then follow it up into a Whimsy [W], then ending with your [E] shield on them. Then, just spam auto-attacks and wait for Glitterlance to come back and keep pounding them into the ground. Also, if you are being chased or pushed hard, you can use the W to run faster and the E to shield yourself, thus guaranteeing a swift escape everytime.

Mainly Offensive AP since you are not going to be going support, but picking up some support masteries is always helpful, and this mastery build seems to work out very well for what you need to do.

Pretty standard AP build for the most part. The Shurelya's is nice to have whether you AP or Support with Lulu. It has good mana/health regen and lets you and your team escape or pursue enemies. Rod of Ages is necessary because being a support champion, she will be squishy so she needs that health from RoA, while also gaining mana and AP from it which is just too good to pass up.