Hi there! This is my first build ever, so I hope you'll use it, because it always works for me. Please don't mind my language,English isn't my native language. I normally don't do much with runes so you can change those always yourself. This is one of the best AP builds for Zilean because he can do great damage with his bombs. Please read everything carefully before judging. And ofcourse leave a comment.

I use mostly magic penetration runes because Zilean's bombs do even more damage if they penetrait. And some cooldown reduction so you can use your abilities more often. You should always use the runes you are the most comfertable with.

Well, there are a lot of spells you can chose, but I prefer Ignite(for the last bit of hp) and Teleport(for defending turrets and keeping lanes). You should just as the runes pick the spells you like and use the most.

That's why you're here mostly: the items. Zilean can be two different kinds of Hero; A carry(AP) or a support. These items are for the carry, so if you're searching for a support build you shouldn't look at this.

Early game:At the beginning you have a mana potion and some extra AP. This is enough to really irritate the enemy team. Zilean has a very good range with his bomb so that's what you should do: Use the bomb. Just use it until your mana and your potion are gone. If you don't have a kill (Which is very unlikely) just recall and get back in your lane.

Mid game:This is the point where you should have got more then 2 kills and Mejais+Sheen and maybe even boots. Now you start assisting your team everywhere on the field with your bombs, Chronoshift the enmy or yourself,Rewind,bomb, Chronoshift. If you've done that you did a fair amount of damage or you got a kill. Also if you're chasing down someone and he turret hugs with less then half hpbar you now can use your Ult. Just turret dive and if the turrethugging person wants to kill you,you put a bomb on him,Ult yourself, come back from the dead bomb him again and chronoshift away from the turret.

Late game:This is the point where you should shine: You got some stacks of souls, and the Deathfire Grasp. Don't forget to use it! Just use your bombs and do the same strategy as at mid game. Now your Ult can save your teammates from ganks, so use it wisely and not selfish.