The goal of AP Zilean is to provide solid early to mid game harassment and ideally capitalize on some early kills. It's a lot more aggressive than a support build which focuses on speed control of enemy and friendly champions. You want to nail down kills early on to have an item advantage for mid to late game. Mid game when you move out of the laning phase, you should be able to damage enemy carries quite effectively from far away and allow your teammates to quickly finish them off. Here's my AP build.

Items:
Doran's Ring + HP potions: Gives excellent AP but most importantly health. You will be taking extraneous damage from minions and you will be focused fired on in the event of an early engagement.
Kage's Pick: This cheap items is part of your Deathfire Grip item. Getting it early will allow the item to pay for itself mid game. It also provides you with a decent amount of AP.
Boots: Just the regular ones for now. Gives you increased speed so you can Timewarp yourself, Bomb, Rewind, Bomb for the hurt hero that's running away.
Fiendish Codex: Again part of your Deathfire Grip build. Provides more AP and CD reduction (10%). This starts to matter more mid game where 3v3s or 5v5s are more common and you will need to utilize multiple bombs.
Finish Sorc boots or Build Deathfire Grip: If you build Deathfire, be sure to put it in a slot you can easily reach (1 or 2). You can left click and drag items in your inventory now which is awesome. Your lead off should be Time Warp, Deathfire, Bomb, Rewind, Bomb. Try to get these skills off as quick as possible.

By now you should have Dorans Ring, DeathfireGrip, Sorc Boots.

Evaluate how well you are doing. Are you getting killed a lot? It may be wise to go support instead and pick up aegis. Are you leading in kills? Meja's may be a the way to go. Are they stacking MR? Then go Void Staff. Mana problems? Go Zhonya's. Other casters in your team doing well (like Fiddle)? Go Abyssal Staff.

Other stuff
For summoner spells I go Ignite + Clarity. Again this is for early lane control and being able to stay in the lane without running out of mana. For Runes I go Magic Penetration, AP, CD reduction. The Magic Penetration early on is especially useful since players rarely stack this early on. Masteries I go 10/0/20 for improved ignite. 21 pt utility is awfully nice but clarity is on a quick CD anyways and you get 10AP for Ignite on CD so its not all that necessary.

im at about 52% wins, so a fairly average elo, and the first time i used it i got a 13 win streak, and i usually get no more than 4.

ghost(optional)+flash(necessary)

amplifying tome + health potion --> mejai --> sorc boots --> zonyha --> then from there i usually get more ap or health depending on how its going. Any other way and you are wasting zileans ridiculous burst damage potential(end game i hit for 900 if i have stacks, x2 and you got 1800 damage within 1 second).


please dont listen to the above poster at least, terrible item build(sorc boots last, wtf??), and ****ty summoner abilities(ignite? LOL!)

As Zilean, you should never been close enough for Mercury threads to have an effect. Plus if they do target you first in a team fight, pop revive on you and watch as your team kills them while they waste time on you.

Also it only takes about cooldown item for Zilean to pretty much be able to use all his abilities anytime.

I use Nashors Tooth because it gives better attack speed, mana regen, AP, and cooldown.

Once I get that, I can spam bombs and timeshift with rewind so often that getting any more cooldown would be a waste.

The fact that each of my bombs can do up to around 650 damage and I can use them so quickly, I can easily lay a squishy down to 1/4 health if they really try to attack me. Combined with the slow, they are not getting away.

Not to say you are doing anything wrong, whatever works for you works for you. AP Zilean obviously works. You should only really need to use support skills when a teammate is in trouble, the rest of the time should be bomb spamming.