I'm not here to learn you how to play Zilean, I just give you my build and why I think it is really strong. The first good point for this build is the cost, except the Archangel, all you're stuff cost less of 2500 GOLD and you're not weak, the Athene give you a lot of AP, this build gives you a lot of peel with quick CDR and a lot of possibilities to heal and protect your mates. You'll rarely late compare with your mates or opponents.

U just need to know one thing, you're REALLY weak in very early, don't try to be a hero. Just protect you're ADC, you need to have your level 6 and you're Godness Tear to be really efficient. In Mid-Game and late you'll become a monster, just stay with you're carries and protect them.